Why does FFMPEG fail when converting Intervalometer photos into a time lapse movie?
The problem is that a sequence of images is not a video file.  In order to convert it to a video format such as h.264, it has to load an entire group of pictures (probably somewhere around 15 of them) and then perform a whole bunch of calculations to determine how the images are related to each other.  It then uses that information to determine what to actually store.  

It doesn't store actual pictures, but rather it stores a couple of pictures (I frames) but then stores information about what changed from those frames to the next frames and what will change from those frames to the frames after (known as P and B frames).  This is a big part of why h.264 compression (and most other modern video formats) are able to fit so much video in to such small amounts of space compared to how much space the images take on their own.

If you were to save it as something like an MJPEG where each image is actually stored on its own, then it would just be a matter of appending them together, but then the file size of the video would probably be unplayably huge.