Has Anyone Replaced a Marathon with a Frontier?

[badmisterkitty]

I have ZERO access to places where I can try out seats in my vehicle, so thought I'd ask here. Does anyone have experience with swapping out a Marathon for a Frontier? Is there a huge difference in fit? We've got 3 across in a Trailblazer and it is currently up against a Chicco infant seat but will eventually be right next to a RF marathon. Both seats (Marathon & Frontier) are 19 inches wide, though I hear the Frontier is a MONSTER seat.

Thanks!