Every time I have tested raw cells under certain amp loads I have seen an increase in thickness of 0.2 to 0.3mm so you multiply that by 4 cells it's 0.8 to 1.2mm and since the cells are already pushing on the case it will distort.

All of this could be avoided if we wouldn't max out the cells thickness but unfortunately competition between factories and brands pushes the limits so cells are now at maximum thickness inside the case.

If the cells get big enough to balloon up and crack open the case then this is a damaged pack and should be disposed of. If the case just rounds up a bit this is due to expansion.