King Bowser Koopa and his castle are from Nintendo's Mario and this screenshot's taken from Valve's Garry's Mod.

Bowser's been planning to try out something new as a weapon and used the magic wand to blimp up his belly. He continues doing so until, outside, he reached the size when his home castle may not be able to contain it. He then victoriously laugh for he would use it in the future Mario Kart racing matches and contests. I think it would take more than just one Mega Mushroom to mach his blimp-sized belly.