I have had many people ask [and have also wondered myself] the physical differences between Frenchies and Bostons. I found online that the biggest difference is in their height.
Bostons and Frenchies weigh roughly in the same bracket, between 10-25 pounds usually, but Bostons are between 15-17" tall while Frenchies are only 12" tall. Frenchies are shorter and squatier, if that is a word, and the squatier the better in my opinion! I compare my little baby to a piglet all the time. :)
I found this piece of art that depicts their differences very well!