Velen is technically the main map of the game. White Orchard, where you started, is the "small" prologue area. So yeah, Velen is fucking gigantic.
But what you're describing is accurate, yes; if you're finding side quests (or quests in general) too high level, too difficult, then progressing the main quest will help bring you up to speed. Remember that quests are marked with a recommended level too, giving you an idea of what is/isn't doable at your skill level (it isn't always accurate, but it's a decent guide).
Wild Hunt is indeed structured in a way that deters "clearing out" areas. Reason being that it's more true open world than something like Inquisition. I too fell into the early pitfalls of clearing out the Hinterlands before moving on, because I didn't want to miss anything (I did ween myself off this though). Wild Hunt is instead a whole bunch of quests and stuff scattered across the map, with no uniformity on difficulty and challenge. So it's not like the bottom half of Velen is easy, and the top half is hard. You'll find easy stuff all over the map, and hard stuff right where you started.
Due to the nature of design it's pretty much inevitable that you will miss something, come back later and find it only to realise you're significantly over levelled and it's a breeze. But in the grand scheme of things there's not a whole lot that gets entirely cut off. A handful of quests become inaccessible after a certain point in the story, but if you just arrived in Velen you're a long way from that.
AFAIK arriving in Velen you basically have three leads to try and locate Ciri. These are your three main objectives, and each will lead you all across the map, and can be completed in any order.
