The size/bitrate of the video is variable based on the amount of motion in the frame, so it may be a coincidence that the size keeps getting bigger, or there may have been an environmental change that produces more noise/motion now than before (different lighting/angle etc),
If you want to automatically join all the 1-minute clips together there are several utilities. The one I use is MP4 Joiner, but ffmpeg can also be scripted to join and convert to a lower bitrate.
You can check out the Wishlist and vote for several requests to allow control over the bitrate. Here’s one that may be applicable: