You've been unlucky. I spent 2 hours last night in a RES being presented with Deadly/Dangerous Pythons, Dropships, Clippers and Cobras. I earned 3.5 million credits. I actually WANTED smaller ships because I'm after kills not credits.
Then you were unlucky too.. getting a "good" RES as you did is less common than the ones with smaller ships. I think if you reset that RES just once you'd have had what you wanted.
I found that the higher ranked ships were more common in low sec system high intensity RESs too.. like Lambayeque
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If you basically just want to farm credit, RES's are still the way to go. You must however reset the instance a couple of times in order for the big ships to spawn though.
**This** just quit to menu and back until you see larger pirates. Then farm that RES till you get bored. You can make 5+mil per hour easily with a Vulture in a good RES.