Complex sentence: While the planned CG21 was much bigger than the DD21, the CG (X) is essentially a DD (X) hull with more VLSs in place of the gun system.
Split into simpler sentences: While the planned CG21 was much bigger than the DD21, the CG (X) was the same, but in a different configuration. Essentially a DD (X) hull with more VLS in place of the gun system.