This is just how I see them all fitting together.
G1(Rescue from Midnight Castle, Escape From Catrina, My Little Pony and Friends, etc.)- Long, LOOONG ago. They were early ponies who haven't evolved into the ones we all know and love yet. Back then they were much bigger because they didn't have as much magic(the most they could do was Twilight's teleportation) and weren't as smart(back then they were as smart as children, now they're about as smart as teenagers or sometimes adults if they're very mature), but after they began to rely less on their bodies and more on magic, the ponies started to shrink along with getting bigger eyes and looking more anime-like. Earth ponies are also smaller because it's the magic that gives them strength instead of muscle. MAGIC MAGIC MAGICMAGICMAGIC
My Little Pony Tales(this is still G1 but very different)- I guess a different part of the G1 world where they are all earth ponies and everything is much more modernized. They are also heavily influenced by the mythical creatures called "humans" and act much more like them than any other pony anywhere(other than Lyra). It's possible they still had the fantasy elements of G1 but the show only focused on the slice of life parts.
G2(like My Little Pony Friendship Gardens)- Ponies continuing to evolve, but again, it's in a place mostly populated by earth ponies. The town in this game reminded me of FiM's ponyville in a cheesy 90s computer game sort of way.
G3(including 3.5)- Rainbow Dash's nightmares.
G4- The best generation ever so far.