what makes bitcoin decentralized is the fact that i can generate a valid public / private key pair on my own and send funds to it without consulting a central authority.
large mining pools are not a problem
large solo miners are not a problem
if anyone big miner doesn't play by the rules that the majority of the network sees fit, the network will simply ignore his blocks regardless of his hashing power
even if it was true that smaller scale mining projects are unfeasible, this doesn't mean bitcoin is centralized.
and the fact is there are many many many small medium and large scale miners, so your statement is false.