Anycubic linear plus arm length

Hi, I have a problem with the arm length.
I made them the exact same size as the original anycubic rods (267mm from center of the Magballs).
The problem is that if I put 267mm in the firmware, it prints smaller objects (on x and y. Height is correct)
To have an object the correct size I have to lower the length in the firmware to 251mm.
But I don't understand why.
Do I have to change the carriages? I still have the original ones 
post your config and configoverwrite.
are the arms parallel?

yes they are parallel, i used duet boards to mount the magballs on the rails.
config.g
; General preferences
G90 ; Send absolute coordinates...
M83 ; ...but relative extruder moves
M665 R133 L251.75 B115 H241 ; Set delta radius, diagonal rod length, printable radius and homed height
M666 X0 Y0 Z0 ; Put your endstop adjustments here, or let auto calibration find them; Drives
M569 P0 S1 ; Physical drive 0 goes forwards
M569 P1 S1 ; Physical drive 1 goes forwards
M569 P2 S1 ; Physical drive 2 goes forwards
M569 P3 S1 ; Physical drive 3 goes forwards
M350 X16 Y16 Z16 E16 I1 ; Configure microstepping with interpolation
M92 X80.00 Y80.00 Z80.00 E96.00 ; Set steps per mm
M566 X300 Y300 Z300 E300 ; Set maximum instantaneous speed changes (mm/min)
M203 X6000 Y6000 Z6000 E1000 ; Set maximum speeds (mm/min)
M201 X2000 Y2000 Z2000 E2000 ; Set accelerations (mm/s^2)
M906 X1000 Y1000 Z1000 E800 I30 ; Set motor currents (mA) and motor idle factor in per cent
M84 S30 ; Set idle timeout; Axis Limits
M208 Z0 S1 ; Set minimum Z; Endstops
M574 X2 Y2 Z2 S1 ; Set active high endstops; ZProbe
M558 P8 R0.4 H30 F1200 T6000 ; Set Z probe type to effector and the dive height + speeds
G31 P100 X0 Y0 Z0.24 ; Set Z probe trigger value, offset and trigger height
M557 R100 S20 ; Define mesh grid; Heaters
M307 H0 B0 S1.00 ; Disable bangbang mode for the bed heater and set PWM limit
M305 P0 T100000 B4267 R4700 ; Set thermistor + ADC parameters for heater 0
M143 H0 S100 ; Set temperature limit for heater 0 to 100C
M305 P1 T100000 B4725 C7.060000e8 R4700 ; Set thermistor + ADC parameters for heater 1
M143 H1 S280 ; Set temperature limit for heater 1 to 280C; Fans
M106 P0 S0 I0 F500 H1 B1 CPartFan1 ; Set fan 0 value, PWM signal inversion and frequency. Thermostatic control is turned off
M106 P2 S0 I0 F500 H1 T60 CHotendFan1 ; Set fan 1 value, PWM signal inversion and frequency. Thermostatic control is turned on; Tools
M563 P0 D0 H1 ; Define tool 0
G10 P0 X0 Y0 Z0 ; Set tool 0 axis offsets
G10 P0 R0 S0 ; Set initial tool 0 active and standby temperatures to 0C; Automatic power saving
M911 S10 R11 P"M913 X0 Y0 G91 M83 G1 Z3 E5 F1000" ; Set voltage thresholds and actions to run on power loss; Automatic saving after power loss is not enabled
; Custom settings are not configured
; Miscellaneous
M501 ; Load saved parameters from nonvolatile memory
T0 ; Select first toolconfig override
; configoverride.g file generated in response to M500 at 20190824 13:02
; This is a systemgenerated file  do not edit
; Delta parameters
M665 L251.750:251.750:251.750 R133.000 H247.539 B115.0 X0.000 Y0.000 Z0.000
M666 X0.342 Y0.141 Z0.483 A0.00 B0.00
; Heater model parameters
M307 H0 A90.0 C700.0 D10.0 S1.00 V0.0 B0
M307 H1 A340.0 C140.0 D5.5 S1.00 V0.0 B0
M307 H2 A340.0 C140.0 D5.5 S1.00 V0.0 B0
M307 H3 A340.0 C140.0 D5.5 S1.00 V0.0 B0
M307 H4 A340.0 C140.0 D5.5 S1.00 V0.0 B0
M307 H5 A340.0 C140.0 D5.5 S1.00 V0.0 B0
M307 H6 A340.0 C140.0 D5.5 S1.00 V0.0 B0
M307 H7 A340.0 C140.0 D5.5 S1.00 V0.0 B0
G10 L2 P1 X0.00 Y0.00 Z0.00
G10 L2 P2 X0.00 Y0.00 Z0.00
G10 L2 P3 X0.00 Y0.00 Z0.00
G10 L2 P4 X0.00 Y0.00 Z0.00
G10 L2 P5 X0.00 Y0.00 Z0.00
G10 L2 P6 X0.00 Y0.00 Z0.00
G10 L2 P7 X0.00 Y0.00 Z0.00
G10 L2 P8 X0.00 Y0.00 Z0.00
G10 L2 P9 X0.00 Y0.00 Z0.00 
@ulcast said in Anycubic linear plus arm length:
R133.000
I would expect that number to change because of the magballs.
what factor calibration did you do?

The real arms length 276mm, but I changed it to 251 to get correct size objects.
I measured R and was more or less 133mm. I measured it again now and it seems to be more 138mm.This is my bed.g
; bed.g
; called to perform automatic delta calibration via G32
;
; generated by RepRapFirmware Configuration Tool v2 on Tue Aug 20 2019 23:53:32 GMT+0200 (Ora legale dellâ€™Europa centrale)
M561 ; clear any bed transform
G28 ; home all towers
; Probe the bed at 10 peripheral and 6 halfway points, and perform 3factor auto compensation
; Before running this, you should have set up your Zprobe trigger height to suit your build, in the G31 command in config.g.
G30 P0 X0 Y99.9 H0 Z99999
G30 P1 X58.72 Y80.82 H0 Z99999
G30 P2 X95.01 Y30.87 H0 Z99999
G30 P3 X95.01 Y30.87 H0 Z99999
G30 P4 X58.72 Y80.82 H0 Z99999
G30 P5 X0 Y99.9 H0 Z99999
G30 P6 X58.72 Y80.82 H0 Z99999
G30 P7 X95.01 Y30.87 H0 Z99999
G30 P8 X95.01 Y30.87 H0 Z99999
G30 P9 X58.72 Y80.82 H0 Z99999
G30 P10 X0 Y49.9 H0 Z99999
G30 P11 X43.21 Y24.95 H0 Z99999
G30 P12 X43.21 Y24.95 H0 Z99999
G30 P13 X0 Y49.9 H0 Z99999
G30 P14 X43.21 Y24.95 H0 Z99999
G30 P15 X43.21 Y24.95 H0 Z99999
G30 P16 X0 Y0 H0 Z99999 S3G29 S1
; Use S1 for measurements only, without calculations. Use S4 for endstop heights and Zheight only. Use S6 for full 6 factors
; If your Z probe has significantly different trigger heights depending on XY position, adjust the H parameters in the G30 commands accordingly. The value of each H parameter should be (trigger height at that XY position)  (trigger height at centre of bed) 
It's very hard to measure the delta radius accurately by hand, so it is best to let auto calibration find it. To do this, use S4 or S6 on your final G30 command on bed.g, not S3.

S6 worked!
It adjusted all the measures and now the object are ok.
Thanks!