I think it is partially that I got better as I played. But I'd expect the worlds to keep getting harder as I get better, in order to keep providing a challenge. I went back and played all the worlds again to try to improve my high-scores; my impression is that World 2 is noticeably harder than World 1, but World 3 is slightly easier than World 2.
I had to buy the Energy Shield and Heart Container in order to beat Stage 1-7. I also had to buy the Glass Cannon at some point, but I don't remember when (I think it was in order to beat Stage 2-7?). The items make a huge difference, especially the Energy Shield. I went back and tried World 3 again without any items, and it was much harder; I had trouble scoring even 25-30 points.
I noticed the crosshair changes when running out of ammunition, but it's a pretty subtle change, and I don't normally look directly at the crosshair itself when I'm playing.