2 TB drive too small for 800 GB

I need to transfer about 800 GB of data from my Mac to my PC, so I got a 2 TB external hard drive thinking that would be sufficient, but it keeps telling me there’s not enough room. I can’t reformat it to make more space because it has to be readable by both computers, and I find it hard to believe that it’s really just the formatting that’s taking up over a terabyte of the advertised space. What am I missing here? Could it be a problem with this particular drive? Thanks in advance!