I can be wrong here but something tells me that MCC sold more than Halo 5, just considering the PC release in the first place. Going by your suggestions Halo 5 sold 4.5m on Xbox while MCC sold only 1m. Doesn't seem likely as MCC had only one year headstart over Halo 5.
Hard to say really. The Xbox release of MCC was badly damaged by how bad it was on release, took years for 343 to fix it. Sales did pick up after it was finally fixed, but late life MCC sales on Xbox didn't seem nearly as good as sales of MCC on Steam or late life Halo 5 sales. I'd actually say that I have the Steam sales of MCC undertracked some, I only said 3m for MCC Steam, but it sold nearly 2m on Steam in just the first few days, so lifetime Steam for MCC is probably closer to 4m than 3m.
We also have Michael Pachter claim that MS told him in 2017 that Halo 5 sales were tracking towards matching Halo 4/Reach, which both sold around 10m, so Halo 5 surely must be at 9.5m by now between the 5m in the first 3 months and sales after those first 3 months, unless Pachter was lying (this is Pachter we are talking about afterall xD).
I might also be overtracking Halo Wars 2 some, which would free up more space for higher MCC sales. I really don't think Halo Wars 2 sold all that well honestly. For one thing, MS stupidly never released it on Steam, Windows Store only on PC, which for sure hurt it's PC sales. Then we have the fact that 343 recently said they have no plans for Halo Wars 3, which also seems to suggest that Halo Wars 2 sold under expectations. Lastly we have the the fact that VGC was only tracking 510k Xbox physical sales for Halo Wars 2 when VGC stopped tracking individual games in 2018, with only 510k physical on Xbox at that point, lifetime Halo Wars 2 on Xbox might be less than 1m, and I doubt Windows Store only PC release accounted for 1m. So Halo Wars 2 is probably more like 1.5-1.7m for now, MS badly needs to port it to Steam.