Hi Aerain,
Stuff like this happened to me my first few games, too (playing Tiny/Small maps with 2-3 AI's to learn the game).
What I found was, my opponents had a better HP capability than me. They usually took +50% HP's for their racial traits, and built shipyards, etc. before they began producing their war fleets. (Whereas I focused on morale/PQ/economic improvements and let my ship-infrastructure wait).
They also were better at harvesting military resouces than me, which is a huge advantage, since the bonus applies to all 3 of your ratings: AF, DF and HP's. For this reason alone, even their most basic ships were far superior.
SB's are also a big advantage in another respect - their military modules add directly to the stats of any ship in their sector. So by defending in their own territory, the AI players were getting big pluses to their AF and DF over me. My ships were toast.
Finally, I was trying to fight with the wrong ship. BC's have good AF, but no HP's to speak of. They're one-shot wonders unless you can build up your shipyard capability. BattleHammers are a lot better. Once I began building those, things started evening out.
Look out for all these things when you attack someone. If you can, build your shipyards first, use BattleHammers, and take out his SB's first. I know they take longer to build, but they survive (and win) better. And if you can get a big planet with lots of production (build your own SB by it and add the manufacturing modules - you don't need a resource to create a SB in a sector...just use the little button on the bottom which has an arrow with a box below it, or press "b", to build a SB anywhere), you can crank out even BB's in a reasonable amount of time (8-10 turns).
Hope this helps.
~SDC~