YouTube automatically transcodes all files uploaded to it's server into it's own streaming codecs, and also creates down-converted files which are lower in resoultion and bit-rate to allow for different streaming options based on the viewers connection speed.
How long is your original video? If YouTube transcoded your native resolution video file from 1.5GB to 250 ish MB, (hypothetically assuming this 1080p we're talking about), then your file is bloated as far as YouTube is concerned.
100 MB isn't a lot of space to work with. And you are not going to find any highly adjustable tools to hit that 100MB mark online. Your options will be limited to either something like Adobe Media Encoder (pay per month), or FFMPEG, command line/learning curve, or VLC, which is robust but not very good with encoding.
If you want to PM me a dropbox link with the 1.5 GB file, I can run it through AME and hit your 100mb mark. It's just a drag and drop, and I'll simply re-encode it back to your shared dropbox folder. Or you can simply subscribe to a 1 month rental of Adobe Premiere which comes with AME, and adjust/play with the settings.