Well, SP's cylindrical family, DCS, is not that thick. It is somewhere in between WASD's super thin caps and GMK's super thick caps. I guess you could say DCS is medium thickness? Also, they are not quite as tall as OEM or Cherry profile keycaps.
What SP is really known for nowadays is their spherical keycap families: SA and DSA, both of which are thicker than DCS, but still not as thick as GMK. But then again, GMK doesn't make spherical keycaps nor do they offer the same ability to provide custom legends like SP does, so there's a trade-off there. I've found that the folks who have a real thing for GMK's thickness have learned to live with the lack of custom legends.
If you want high-quality, thick cylindrical keycaps then hold out for a GMK set; if you want high-quality, thick spherical keycaps then hold out for an SA set (or jump on the amazing /dev/tty keycap set that just went up for sale on MassDrop today).