I mean...even when the diplomacy window shows them as "equivalent" with a much smaller bubble than i have...when i attack them they usually have a much larger fleet that destroys my fleet while a smaller fleet of them conquers my planets.
am i missing something here?
im pretty much used to micromanaging fleets, which isnt possible here.
im also a player who uses a lot of intel, scout the enemy, see what he has and where he has it...which isnt possible either.
so...how does war work? how can i be victorious?
To answer your question, war is a funny mechanic in Stellaris to say the least. How it's used and why are extremely subjective from game to game, but in Stellaris it's fairly straight forward in being solely about conquest. Either you overpower your chosen enemy through military might or they you, the end result typically being either total victory or defeat.
If you're talking about "smaller bubble" as in borders then my best answer would be don't judge a book by it's cover. The number of systems a foreign empire controls can be very misleading. There's much you don't know or have access to in regards to assessing the enemy's make up. Which from a commander's stand point would make Sun Tzu weep in anguish. The number of mining stations fueling their war fleets, the efficiency of their population (affected by happiness), and the extremely broad and un-informative total fleet power comparison in the empire communication window. Then to top it all Stellaris has a fairly lopsided rock paper scissors triangle of missile, ballistic, and beam weapons where beam cuts armor, ballistic pierces shields, and missile just sort of foolishly crashes directly into whatever is being shot at. Then there's the giant killer torpedos designed to offer smaller ships an advantage over battleships and cruisers, hangar bays which have remarkably predictable flight paths for their fighters, and the shining glory of all spinal weapons in 1.3, the Tachyon Lance, scourge of the Prethoryn and bane of the AI uprising.
So to address the strategic side in outmatching your fleer there're... some things you can do? Re-balancing your fleet design... kinda helps? It's extremely limiting from the micro perspective (I play starcraft 2 off and on, I feel your pain not being able to control my fleet's behavior like kyting and concave advantage) but it does it's job for the most part. Instead of having diversified options for . I really wish there were a skirmisher option but with my limited time of play I haven't really found any.