For wake separation purposes, the 757 is classed as a Medium (based on weight), even though it produces vortices comparable to some Heavy types (in the UK it's classed as an Upper Medium for that reason, to differentiate it for wake separation purposes from 737s, A320s, etc).
The above reason is why on APPROACH 757's should be treated as HEAVY for separation reasons, as it is in this configuration when their wake becomes troublesome to a following aircraft when classed as MEDIUM.