He's crawling on his stomach, with a pack on his back and almost no clearance between the top of the pack and the bottom of the carriage. The small-ish trapdoor hole is in the middle of the carriage floor, and he can't roll over because, pack, so he's got to arch backwards to get up into the carriage, which is made more difficult because of the pack. It's not that he can't fit, it's that he's not necessarily that flexible. :D