To the OT:
See if the comfort pad is bunched up under the seat area. I one thought DS1 had outgrown the MA by height at almost 3 but the pading wasn't at the right harness slot and had slid down and bunched and folded on itself.
Totally off-topic:
In my experience some brands of girls shirts run much smaller than their boy shirts.
My DD has to wear a 7 or 8 in some brands because of them being too short and tight in the chest and she is usually a 6 but would swim in the same brands size 7 or 8 boys shirts. It irks me so much that I have not bought things for her if I didn't really need them and went to a different store. It drives me bonkers to have to buy her a size 8 when I am buying her big brother who is 24 lbs heavier and 6 ins taller a size 8. I don't think I would have noticed it if I wasn't shopping for multiple kids in the boys and girls department at one time.