I thought that too until i cut up my RM125T pipe for my RM175EF project and decided to rebuild the RM125T that had a 100 crank and top end.
I rebuilt it back as a 125 and fitted the other pipe that i thought was off the RM125T or the 100X that came with the bike.
The exhaust outlet at the barrel is smaller on the 100....so to answer your question,same shape,smaller at the header diameter for the 100 pipe.
