I made a thread in general about it and it seems like the reinforcements that happen as a random event scale to wealth while the reinforcements you pay 25 goodwill for do not. While a difference in power may come down to balance, this is clearly a mistake, as one user reported that their random event friendlies had 80 allied pawns while the called-in friendlies had 9 pawns on the same map.
Given the fact that these friendlies require a very notable payment, their sheer uselessness implies this is a bug. In many cases they are unable to harm the enemies they were called in to fight. This makes diplomacy feel quite underwhelming.