The factor directly linked to video file download speed is the bitrate that the video is rendered at, not the file size in MB. The bitrate is the amount of data which must flow into the video player to display the video without interruption or buffering (the choppyness you are talking about). This bitrate is set during the rendering or transcoding of the video file.
Broadly speaking you should choose a bitrate between 250kbps and 500kbps for a 320 X 240 (screen size) video. Increasing the display size will require a proportional increase in the bitrate to maintain equal quality. About 250 is used for the standard broadcasts on YouTube, not for quality purposes but to ensure somewhat reliable downloads. For better quality video I recommend trying to get closer to 400kbps. However, even with my 6Mb fast Internet connection, there are times when some of my 500kbps videos suffer from buffering due to server load and other Internet traffic. So while in theory a 1.5Mb connection has nearly 4 times the bandwidth needed to deliver a 400kbps stream, all the overhead and other Internet traffic can drastically cut the actual delivery capacity.
So the real issue is not the file size in MB, it is the bitrate of the video. The file size in MB only affects the length of time it takes to download the entire video. And if the server is progressively downloading at an acceptable bitrate, the video beginnings playing almost immediately and the downloading continues unseen, as the video plays uninterrupted. So for example the Internet connection is downloading at 500kpbs while the player requires only 400kbps to continue playing.
Eye for Video