the MTOW for heavy aircraft was lifted from 255,000lbs to 300,000lbs roughly 2 years ago by the FAA to conform with ICAO standards.
The reason being was that a B752 being at that exact weight could exceed that weight while fully loaded, but not exceed it when it arrived at its destination. therefore, controllers would be confused as to how many miles of separation were needed between that aircraft and the one following it (ATA’s B752s and all B753s were exempt from that issue, as those aircraft were already configured to exceed that MTOW limit). Now that the FAA is conforming to ICAO standards, all B757s are underneath the MTOW limit.
In the case of the C17, it had to have exceeded the 300,000lb MTOW limit for it to be called heavy.