Sorry everyone. I can't stop toying around with creature of mine. Unfortunately more I play - more mistakes I discover.I barely was able to put in that chair (not because he's fat, but because technical stuff, which I needed to correct).
Anyway I just need to improve concept of working with vertex weight mapping... the way I worked tend to induce a lot of errors and screw things up in general.

But whatever - look! He squeezed into that office chair :)
Actually that chair is nice because it only restricts hips area, which is not that big on him. Rest of the body may be enlarged with no consequence to get stuck.