Reaching the maxPoolSize from the minPoolSize means creating the connections at the crucial moment where the client application is in the desperate need of completing an important query/transaction which the primary responsibility since it cannot hold the data collected.
So here the connection creation action is the costliest among all the other management tasks. so keeping the connections ready is the best option.
poolSize parameter is very good in the sense when the application owner know what is the optimal number to put, after having application performance analysed with the history of previous settings and the improvements made on it. server sizing always shows up in this sort of analysis.