Is Chromoly Steel heavy

Depends on what youre comparing it to. Cr-Mo is often held to be the "best" steel alloy for things like bicycles.(do note that there are different kinds of Cr-Mo available) Next step down the performace ladder is Hi-Ten(high tensile) steel.

By volume there are virtually no difference in weights between the alloys, but Cr-Mo is stronger, so you can build an equally strong structure using less material, and get a lighter finished product that way.