It's not about the budgets, but about how they are used.
GENS feels like a TV episode because it largely
was. They commenced shooting not more than a few weeks after TNG ended (in fact, there was a crossover in filming, but that's because the Enterprise-B segments were going in front of the cameras at the same time TNG were filming their last handful of episodes), and therefore it carries a momentum from the series. They did what they could to differentiate it, like the changes to the bridge and the lighting, but given the short ammout of prep time they had, it was never going to be
quite as cinematic as one might hope.
FC remedied this, but in many other ways it was shot like a TV episode. Not for nothing was the director nicknamed 'Two Takes Frakes' for his ability to get footage in the can with an effeciency bordering on the miraculous in movie-making terms (in reality, this 'miracle' was simply that Frakes was used to shooting television, and carried that methodology over to the big screen). Where FC "feels" more cinematic is in the script. The stakes are high, the Borg are a suitably gruesome cinematic villain, and there are some terrific set pieces, like the Borg cube battle it opens with or the space walk on the hull of the Enterprise. So, while it was cheap to make, it looked much more expensive on screen.
INS is the obverse of this. It was given a bigger budget, a
much bigger budget than First Contact, and yet many people think it feels less cinematic. Why? Well, the reality is that the budget went on extensive location shooting and the hiring of hundreds of extras. While that does mean the budget is spent effectively, it's hard to really grasp the enormity of that in the finished product, because the script is much more low-key. It's filmed with these grandiose locations and in big gestures, but the story itself could have been told in a 42 minute episode and shot on Paramount's 'planet hell' set, and still not felt like it was missing anything...
NEM... is a movie. It's maybe more cinematic than any of the others, both in terms of how it was shot and in terms of it's story. Most of what it does is BIG BIG BIG, but to many people this meant that it felt too far removed from the subtle intimacy they loved so much in the TV show. Go figure.
