That is only a first order approximation. Second order effects scale badly. Larger tanks for the same amount of propellant have more weight, which means they need to be built sturdier, which means even more weight, etc. LH2 can’t have a common bulkhead with LOX like Falcon 9 has, so that’s more weight, and sitting on top of the tank so the tank needs to be studier still. And LH2 is way lower temperature so you need foam insulation, which adds weight. And you don’t want that foam peeling off and hitting the SRB on the way up, so you need a layer of sealant, which (all together now!) adds weight.

Etc. etc.