Odd coincidence this - we live in an area with the exact same set-up. Any time we mention the possibility of changing these percentages the reply from the Managing Agents is invariably "It can't be done - the percentages are written in stone because you signed for them in your contract."
Now naturally I don't believe something can't be changed - it just takes the will of the right person to change it, if you could establish who "the right person" is.
However, one of my neighnours made a valid point when we were discussing this. If the overall budget for the whole area is €X and it's apportioned out according to the percentages originally agreed, then if you successfully petition to have the percentages for 2-bed duplexes reduced, the overall budget remains the same and so this reduction has to be spread out among other property-owners. Which means increasing the percentages for those who own one-bed apartments etc. This, I think, is where the problem would lie - the owners of 1-bed apartments can come back and say "our percentages were written into our contract and cannot be increased", and legally I think they'd be right.
So I think the only way to get the percentages changed would be if all residents affected by the same budget agreed to the changes, even though some would be going down and some would be going up. Can you imagine the fun you'd have trying to get a resident to agree to have his/her management fee increased?
Regards, Liam