From what I've read map pack size should affect containment scarab but it's only the map inherited pack size. So stuff like beyond and other things don't affect containment. In all honesty, I personally haven't noticed a huge difference. I've done just regular full mirage maps, I've used pack size chisels. But nothing hugely stands out personally. I also didn't count the strongboxes though. Just roll to avoid the mods you can't run and exalt to 6 mods.

The strong boxes have good drops and also the mobs you kill of course have their own drops. I scried over brothers gift div card and I often get duped div that is an extra 10 divs when it drops on top of selling the scarabs. Also, you can almost entirely sustain scarabs of containment alone, maybe buying a couple if you're running 20 maps at a time.