Well, tbf, we have about 9 times greater population than Bulgaria, and just over 4 times their MEPs, giving us a representative rate in the EUP which is 5 times greater than that of Bulgaria.
The details of the revised representation formula are interesting, but the ultimate effect is that we have greater influence in line with our greater population; in fact, the core-EU always had a proportional advantage over the newer members; but if you consider how the voting blocks align regionally, then the picture and the formula produces a very balanced result.
But let's assume the hypothetical ascension of Turkey after XXX years; what happens then? Well, they would have a decent chunk of MEPs, but in terms of regional block-voting power they will be quite isolated, unless they can find a way to smooth out relationships with Greece, Cyprus, the Balkans, etc; and even then the core-EU remains in control, really.
A better proportionality rule can be devised, yes; but if you are not willing to rule out the very unlikely event of Turkey joining, why are you ruling out the more likely event of the subsequent EUP reform? From what I understand, the formula was last revised post-expansion; why not again?