I used to have a chunky monkey and he was in size 6 diapers(could have been 5 but mom always bought big) at around 16 months. Mom and I used to always joke that he would have to be potty trained well before he was ready just because he wouldn't fit a diaper. He eventually started to thin out (I think he actually just got taller lol) and he did start to potty train early on his own! He left my dc before he was fully potty trained so I'm not sure what happened but I imagine he was trained shortly after that.

The underwear over the diaper is probably a good idea!