I saw the first two Gigantours at Pine Knob. The first one had a pretty good crowd, but even then, the seats didn't look full until Megadeth themselves went on. The second one was much, much emptier and the bands were commenting on how many empty seats there were. The actual number of people was very respectable - they just booked a place that was outsized. It didn't look full until Lamb of God went on.
Megadeth is a Royal Oak Theater or State Theater-sized band, Dave just won't accept that.