Grapplers are ''thick'' because grappling requires lots of physical power and training grappling with a partner easily provides enough muscle stimulation to make you grow ''thicker''.

Anyway I don't see what your question really is.. If it's asthetics then I don't know since I don't care about them and don't think any need to exist in martial arts - at least not in the forms most see them in.

In terms of functionality our bodies and body types are different, muscle gains and such are affected by how we train and how we train is affected by what we're training for. I could go on a long long rant BUT in the end of the day to summarize; the main thing to any martial art is body mechanics without good body mechanics power & speed are lost. You can be ''bulked up'' and do good karate provided you have good body mechanics and vice versa be skinny and perform well.

So it doesn't matter if you train in the right way focused on your goals and requirements your body will be what it needs to be.