There is no way 5.10.0 is using twice as much memory as 5.8.6 under normal circumstances. I would strongly suspect that you're using a 32-bit 5.8.6 and a 64-bit 5.10.0. Either that or you're using an old version of Devel::Size that doesn't know about the new SV header arrangements in 5.10.0 (which in general use less memory than 5.8.x).

Dave.