[Freesurfer] problem on mri_vol2surf

I have tried to map the functional volume onto MRI surface

functional image has been coregistered with MRI (voxel size is resample to same size)

Here is the command

mri_vol2surf --reg forig.mgz --trgsubject 003_S_1257 --hemi lh --surf inflated --surfreg lh.sphere.reg --out lh.fsurf.mgh --srcreg forig.mgz.reg

forig.mgz is functional image 003_S_1257 is MRI

orig.mgz.reg=[003_S_1257 1.000000 1.000000 1.000000 1.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 1.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 1.000000e+00 0.000000e+00 0 0 0 1 round]

After surface generated, we found that generated surface is smaller compared with original MRI.

Did I make any mistake?

thanks Lan