I liked the size increase/decrease mechanics, and how to made use of it for the platforming.
One thing that frustrated me, however, was the lava spawn/last checkpoint reseting, which I think was a bug and not intended. However, I was able to get to the end.
If you polish those things and tight the controls a bit more, with time, after the jam, and maybe add more levels to explore the mechanic it could be a nice platformer.