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.
I would also add that any discussion about giant size changes should consider
multiple editions.
BECMI: (rule cyclopedia, the B/X version is the same size)
Fire giants are large (16 feet tall), they have 11d8+2 hp, do 5d6 damage and have an AC (converted to ascending) of 16
1ed AD&D EDIT: I found the book, and I'm glad I did!
Fire giants are large (12 feet tall), have 11d8+(1d4+1) hp, do 5d6 dmg, have an AC of 17 (converted to ascending)
2nd ed AD&D
Fire giants are huge (18 feet tall), they have 15d8+(1d4+1) hit points, do 2d10+10 damage and have an AC (converted to ascending) of 21
3.5 D&D
Fire giants are large (12 feet tall), they have 15d8+75 hit points, do 3d6+15 damage and have an AC of 23
4e (essentials)
Fire giants (base) are large (exact size is ?), they have 174 hit points, do 2d12+13 damage and have an AC of 34 (!)
5e
Fire giants are huge (18 feet), they have 13d12+78 hit points, do 6d6+7 damage and have an AC of 18.
(one should also do an analysis of
size rules in D&D, what the cutoffs were and their impact on combat in general, not just giants).
So we see an increase in power and size from B/x to 2nd ed, with the giants having more HP, better AC and doing a bit better damage.
In 3.X, giant size
decreases but their AC improves, they do more damage and they have a lot more HP.
the 4e giant's size doesn't change too from 3.x, the main change is a massive improvement to AC
in 5e the HP stays the same, the damage barely creeps up, the HP stays the same (despite 5e having "HP bloat?") and the AC is adjusted to reflect the bounded accuracy of 5e. However, they return to their majestic old school size.
SO IN CONCLUSION:
3.x and 4e giants were abnormally small.
EDIT: the addition of 1e AD&D adds an interesting curveball to the analysis. Apparently giants have gone through TWO cycles of shrinking and growing! Interestingly, both rounds of shrinking were accompanied by a rule complexification (AD&D, 3e). However, the AD&D giant wasn't significantly more powerful than the previous one, while there is a significant power just in 3e.