They measure (with a ruler) within 1/2" of eachother. I think that the MA measured slightly more, but the MR seems to have a tad bit more space since it curves at the lower spine so the child fits differently. Here is dd around 41.5" tall and 34lbs and 3.5 yrs old. She was looking up in the DC and looking straight in the MR, so it is a little hard to compare.
DD in DC/MA ff:
DD in MR rf:
In my estimation, she would out grow the DC/MA by height and weight nearly exactly turning 4 yrs old. At the same time, or perhaps 6 months later, she would outgrow the MR by height. By 4 she'd also outgrow the TF rf by weight.
As a side note, the MA/DC has less leg room, but it has more room for the child to cross their legs. The MR has a deeper seat, but less room for criss cross apple sauce. The seating area of the MR is rectangular with high sides, whereas the MA/DC is more of a V, and is wider and lower at the sides where the child seat hits the seatback.
As another side note, I love the way the MR head coverage gets taller and goes all the way up to the top of the seat back, vs. the MA that gets lower and lower and then disappears. You can tell what I mean in the pics.