I don't think any of them get up to 75 feet high, that's probably the principal difference. Many of them last well into July, though, so I wouldn't yet say that the Boston one will last longer.

Salt content is probably also a significant factor. At least here in Ottawa the roads are very heavily salted. You need to salt much heavier to melt ice at -30C than at -10C.

If you look at some of the pictures they've got very heavy bulldozers pushing snow up the hill and excavators moving it around. It would have gotten heavily compacted as the bulldozer drove across it. It's probably more ice than snow at this point.

I don't think we pile it so high, though I have never worked in whichever civil service branch handles snow so it is only based on life-experience.

I have never seen a pile so high, we have many smaller piles scattered throughout our cities: every parking lot has one, the parking lane of most roads becomes places for snow piles, just outside of town you can see fields of small piles. The snow that is actually removed from the roads, I assume they spread that out into smaller piles as well.

Those pictures of loaders on top of huge piles of snow seems particularly dangerous.

Wild guess: Boston is a densely populated city with geographic restrictions (due to the bodies of water around it). The ratio of roads needing to be cleared vs available snow dump areas might be a lot higher than in most places in Canada. Also this was a very extreme snow year for them, so it wouldn't make sense to set aside this much land for snow dumps every year.