TMBTM said:

but I guess it is kind of ridicules that it took about 20 years to make the first one and like 3 years to make the 2nd bigger one

Mmm, maybe we could say that the first Death Star took three years to build also. It is just its "destroyer of world" ultimate weapon that is not fully operational yet at the begining of ANH. Maby the Death Star was a secret base for the Empire for a long time.

I am not a Lucas apologist here, I just try to (desperately) find a solution.

"Until this battle station is fully operation, we are vulnerable" and that was days before it's first firing on Alderaan.

The only way of keeping the Death Star being built in ROTS is to have the second one also being built roughly about the same time, Death Star One, is Fatman to Death Star Two's Little Boy ( I mean in terms of when they are built not in size as DS2 is meant to bigger which would also slow it's building down).

Resources may have been focused on getting the first one finished at a later stage, halting progress on the second one and using the first Death Star to iron out the bugs in the systems.