Withcer 3 only has a handfull of quests that are not tied into main questline sure you can skip a lot of them but they are mostly revolving around main story, monster hunting contract(which are basicly all the same with diferent targets) and treasure hunt ones so i wudnt rly use Witcher 3 as an example quest wise cuz apart from the rly good main quest there was not much else inthere.
Now about comparison to Skyrim(and we are talking vanila Skyrim here right, no DLCs and mods since Fallout 4 doesnt have those jet) i am rly not seeing lack of things to do in Fallout 4 i am over 100 hours in and i havent rly done any of the main quest past arriving to Diamond city i did however do any sidequests i ve found around and while many of them are not on obvius locations(you can get em from letters found on bodies in the wasteland, from radio signals, after entering certain buildings and interacting/speaking to someone/something etc..) and i have so far cleared mybe about 60 locations(did not jet do any of them twice i do know later on radiant quests will take me to respawned locations sometimes but thats no different than Skyrim), so to sum up 100+ hours in at least 70% of the game left i wud def not call that less content than Skyrim.
There is some diferences though all major questilines are tied to factions which means tied to main story so after you chose a side and finish main story you wont have any non radiant long questlines left to do(hopefully DLCs and mods will take care of that), this being said though that doesnt mean you have nothing left to do after you finish the main story unless you explored thurally and did all the side missions already(however to do that i reckon it wud take at least 300 hours which in my opinion is rly good leinght of the game.
Also i dont know which dificuly did the OP play on, cuz ofcourse at lower ones you can simply blitz trough everything faster and i do think he did miss a ton of discoverable side quests/missions plus compared to Skyrim Fallout does have a higher empasis on exploring so if you leave out that and stick to locations you get sent to via main story you will miss on most of the game and on plenty of optional content.