More tough than the average Koopatrol, the dark ones seem to not work for Bowser. And it seems like despite their strength they too can end up at sizes like this. But given their strength, I highly doubt this guy is even hindered by the weight. They probably just took a moment to sit down, and could somewhat easily get up at any point.

Of course I had to make an alt to the regular Koopatrol. It only made sense to do it