The Esslinger motors have a few more inches because they are actually a production-based design. The ST (and EST now) uses lots of stock parts and dimensions. The CORR/TORC/Lucas Pro-lite guys run a very similar motor on gas with an iron block and they make similar power. The aluminum block is a weight and reliability thing.

Since they are, they have a smaller (much smaller) bore center than the "half a V8" designs. They can't run anywhere near the intake valve size ( or should I say utilize?) that a 4.something bore motor has room for. You can put a bigger one in a small bore, but the cylinder wall shrouds the port.