The membership fees are not the main source of funding of FYEG, so there is no need to harvest more money from the membership fees. In particular, if a progressive system increases the membership fee for a MO above one percent of their budget, it would drain money which they need for their basic political work. Moreover a fair calculation of a progressive fee would have to take into account the real purchase power of the MO's resident country. This is difficult to evaluate and would make the process much more complicated.
some background information on the main sources of administrative income of the FYEG: EGP contribution – 3,5% eligible costs: 52'000 €; CoE European Youth Foundation 16'964 €; Erasmus+ Admin: 35'000 €