take shoulder straps off and you will get tonnes of room. with the mb if you have it all the way up the back of the seat its shortened. or lengthen the straps and pull the nest down iykwim.
my ds is a big 3yo(3 next week) and still has bags of room. he actually takes his shoes and socks off in it as he likes the fleece on bare feet. but he's a bit weird sometimes.
if you have a bumper bar on the mb you could do away with shoulder straps altogether