If yes who decides that? Does the algorithm have rules to change that automatically? Will miners approve that if they get penalized?
The miners essentially decide (with bitcoin the rules are sort of obvious so everyone agrees to them) but with multi-pow coins miners have to rely on working out what will make them the most money/coins. In general this means sticking with what everyone else is doing, the only practical way of knowing what everyone else is doing is assuming everyone is following the software. In theory thus, optimum strategy is to follow the software and hence the developer gets to choose.
In reality, some miners might refuse to download new updates. Now you have different miners enforcing different rules, working out the optimum in this situation is far from trivial and could lead to instability. The full economic analysis of this is very complex.
Fair point, we are working on a dynamic method of weighing although the fix worked thus far (8x hasing power increase within minutes on SHA algo did no damage at all to the chain and a few weeks earlier a scrypt pool with 51%+ forked resulting in just that their miners lost some mining time again with no consequences to the chain) we are aware that fixed factors are somewhat unreliable in long term.