In the Ethernet CSMA/CD algorithm, inter frame gap (IFG) time is 9.6 microseconds. When the number of bits in my frame changes, does the IFG time change as well?
Nope. The next device to transmit a frame always has to wait the Inter-Frame-Gap period from the end of the last frame before beginning the preamble of the next frame.
It's not like Ethernet is made up of a bunch of fixed length slots for frames. Some other data link layers such as ATM's notion of "cell switching" might do that, but not Ethernet.