Isn’t the sound directly proportional to the weight of the transformer & tube used?No- that simply has nothing to do with it. Things to look for: a simple signal path, available tubes (some preamps use tubes that are no longer made), whether or not the unit has regulated power supplies, construction quality.
If you have a simple signal path its likely that the circuit will be more transparent with wider bandwidth. The power transformers simply have to provide the needed voltage and current such that they can do the job. Any excess capacity that the transformer might have doesn't affect the sound- it simply means that the product is heavier without other benefit.
Now damping **can** affect the sound as it reduces microphonics and a good damping system does add weight.