Well, this archetype certainly raises a lot of questions...
1) Bonus known spells... again? Didn't they go quite far into explaining why it was such a bad idea that they had to remove bonus known spells from the Storm Sorcerer? I am all in favor of increasing the known Sorcerer spells, but I'd like WotC to take a clear stand about it.
2) Why exactly having a Giant ancestor would be a "sorcerous origin" i.e. the source/reason of your innate spellcasting? Only cloud and storm giants can cast some spells! All other giants are just big brutes with nothing magical. By that reasoning, we could have a "human soul sorcerer" too.
3) Assuming it's now become officially ok to grant bonus known spells, and that it's acceptable that a non-magic ancestor causes magic descendants... at least the bonus spells could be the same spells that such ancestors can cast, not something else like invisibility.
4) Clearly, this archetype is all about investing in a high Constitution score. That's interesting, but a bit odd... are they trying to create subclasses that have a higher ability score cost in exchange for more power over the baseline? Like, we give you bonus known spells which are normally unavailable to sorcerer subclasses (and NONE of your subclass features will cost sorcery points, yay!) but we make you pay by making you more MAD?
5) The ability to increase size is cool. But it sounds like something you'd want to do when you're out of spells, and thus decide it's time to wade into battle! So it makes no sense at all that this ability is triggered by casting spells. It could be more simply an action or bonus action of its own.
And on a more general tone... again with more subclasses?? When are they going to work on something else? :/