First of all it obviously depends on the video resolution. Lower resolution video takes up less space, but you probably know this already. Same is true for the frame rate (frames per second). Other than that, it mostly depends on the video compression used.
When saving or exporting a video, you can choose between several different file formats (for example MOV, MP4 or MKV). Each file format can accommodate a number of different compression methods (AKA codecs), both for video and for audio. Some codecs are better at compressing video than others; some are better at preserving the image quality than others; some are faster at compressing than others; and some allow you to adjust all kinds of parameters to make it faster, slower, better quality, worse quality, etc.
The most commonly used video codec these days is H.264 (which you can have inside any of the 3 mentioned file formats), which is also what the camera itself produces (i.e. the video files coming straight out of the camera are H.264 compressed video). H.264 can do a number of things for you, depending on how you configure it. First of all, the image quality is adjustable. If you lower the image quality, the video will look worse, but the files will be smaller. Higher quality increases the file size.
Then you can adjust all kinds of compression features, normally done through codec profiles. Those basically affect how hard the codec tries to compress the video. The result is that encoding (saving, exporting) the video takes longer to complete, but the file will be smaller at the same image quality. Or you can speed up the process, giving you a larger file at the same quality. This can go to an extreme, where it can take hours to encode just a few minutes of video. This is how you get your high quality full length movies with a pretty small file size, it just takes forever to encode them.
I know nothing about WMM, so a generic explanation is all I can offer. You should be able to figure out which codec WMM is using and possibly make it use a different one. You may have to install some additional software (codec packs or w/e) to make H.264 available to you.