. . .and I'm also wondering, does WotC just arbitrarily change the size of creatures to encourage players to buy new minis to accommodate? That's what it feels like. How many other creatures went from large to huge, or vice versa, or maybe from medium to large?
They changed the size of giants for 5e because they wanted them to be truly gigantic and of epic proportions, not because they wanted to encourage people to buy more minis or to make previous minis incompatible with the new game.
With 5e, WotC has de-emphasised the use of miniatures. In fact, they don't even make the minis themselves anymore. They've licenced miniature production out to WizKids, who have ironically been somewhat inconsistent with the 5e mini sizes as well.
WizKids' first two 5e giants -- a stone giant and a frost giant from the initial Tyranny of Dragons set -- were Huge minis posed in such a way as to fit onto Large bases. The ogre from the same set also looked like it should be Huge but was still on a Large base. Subsequent ogre minis have gone back to being around the same size as the one that was in one of the Dungeon Command sets.
WizKids also messed up the sizes of their initial elementals from the Princes of the Apocalypse set. According to the 5e MM, the four basic elementals are all supposed to be Large, but WizKids's water and earth elemental minis from that set were Medium. (I note that they included all Large elementals in the most recent Fangs & Talons set.)
I would also argue that most of WizKids's 5e-era dragons are too small, even the supposedly adult ones like the silver dragon that was a premium figure for the Princes of Apocalypse set. They now seem to be attempting to rectify this by producing truly gargantuan premium dragon figures, like the big red, white and black adult dragons, and Arveiaturace the ancient white and so on.
Incidentally, I have some old Large-size 3e era giant minis (including a fire giant) I've set aside to get rid of, but I'm in New Zealand, so it may not be a feasible option for you ...