not really my amp for example is a rockford bd1001 amp and it is really small an weighs maybe 7 pounds or so and it puts out and underrated 1000 watts. cheaper amps maybe twice the size and do not put out half that power. the thing to look for with amps are the reliable name brands such as jl zapco kicker certain rockford fosgate models and jbl amps there are other but thats just to give you an idea
U just cannot judge any amps with that measure - it depends on the quality on the components which comprise the amp thats why a 1000wrms amp will cost $200 in 1 company(like Boss) & $1000 in another like Zapco .
no because you don't know where the weight is.. power supply, case, heat spreaders, etc..
also the class of amp will greatly affect the footprint and generally the weight as well.. a class D amp is smaller and puts out more power than a class AB amp generally speaking.. at least per ampere it does.
it's better to see how the amplifier is put together instead.
Unlike home stereo that contain large transformers, car amps don't. If people judged amps' quality based on weight, then all the cheap manufacturers would simply replace anything aluminum with metal/steel case and add $5 10lb weight inside.