Excuse me I do not understand..  How or in what maner that is giving power to central authorities ??

Maybe that the non popularity and mining difficulty could open a way to some parties to put influence on it, but what is the correlation between bthe blocks size and this ?

Spamming network could be stoped by simple parades like for example deciding a reasonable minimum fees price ..

Historically have chosen not adopting the block size increase (which in implementation figuring on the original code of Bitcoin) just to assure themselves a good revenue from fees!  Even Gavin Andresen was very partisan to the block size augmentation and just gave up from BTC after that episode!