Did a test to combine two VOB files.
2 x 330782 = 661564 compared to the final size 661558
So I would say no. If you get a considerable higher size difference it could be that a stream (language/sub) is removed based on your settings in Nero Recode.
Do be aware that there is a bug in the merging function. The time table is not 100% right after the merging. To solve this, open up the merged folder in PgcEdit (freeware) and let that program automatically fix it for you. (You just need to open the program and save the files).