Jovica,
Not sure why half of the FCA 757's fall into Heavy for you. One possibility may be that your ACC coding software might be using weights supplied by Eurocontrol for navigation charges etc. The 757 fleet have a range of different aircrfat max weights available (all the same aircraft in reality, but when you order from Boeing you specify a max-take off mass up to their absolute max 0f approx 113,500kg - and PAY for the more the paperwork certifies you to!).
However even at max, the 757's still fall into ICAO's Medium category - and this is what is filed on FCA flight plans. Does Serbia use a differnt split from ICAO at all? The US as pointed out takes the 757 as a Heavy because of it's vortex, and the UK further complicates issues by splitting the Medium into 2 seperate categories!!
Thanks for your en-route service by the way - it's always a pleasant transit!
30W