The Constitution Class (NCC-1701A/B) were effectively light cruisers by the TNG era whereas the Galaxy Class was a super-heavy explorer cruiser. The main difference between them was the NCC-1701 was designed to operate for 5 years without service but the NCC-1701D could theoretically run indefinitely on its own without major battle damage. Large part of its mass is form the power systems needed to run its own industrial fabricators (replicators) and the experimental research hardware. That and all the families living aboard.
The Constitution Class (NCC-1701A/B) were effectively light cruisers by the TNG era whereas the Galaxy Class was a super-heavy explorer cruiser. The main difference between them was the NCC-1701 was designed to operate for 5 years without service but the NCC-1701D could theoretically run indefinitely on its own without major battle damage. Large part of its mass is form the power systems needed to run its own industrial fabricators (replicators) and the experimental research hardware. That and all the families living aboard.