Video edit question - quality loss with increased file sizes on export?

ThePrettyDamned

I'm screen capturing at 30fps, native resolution (2560 x 1440) in .flv format. It looks great when played back in VLC. 

This then gets converted to a 1920 x 1080 MP4 via Handbrake. After some experimentation, I've been able to get excellent results from this - crf is 20, ad I'm encoding at 'Very Slow' speed, which provides better quality in smaller file sizes (I don't mind being patient, my videos won't exceed 30 minutes max I should think). Again, quality is bob on, despite the reduced resolution, text and lines are sharp.  

Then, I drag a few videos into WMM. The previews are shite quality, but that's fine - apparently it's sometimes like that, depending on the files it's looking at.  When exported, I export at 30000kb/s, full resolution (so 1080p) and I'm getting file sizes approximately DOUBLE what goes in, yet an overall loss of sharpness and clarity top the image.  This is higher than it's 'For HD Display' setting - which defaults to 24000kb/s. 

It's not horrendous but it is there.  I'd say it creates an acceptable result, bar the fact it is obviously compressing the file yet winding up with a larger file at the end! So basically, is WMM not doing it's job properly?

Yes, I'm aware YouTube offer another layer of compression but that's okay - I just want to feed YouTube the highest possible quality I can. I'm happy with everything up until Windows Movie Maker, which is a great little program to intuitively stitch clips together, and even have fade ins/cuts etc. 

If this is just WMM being a bit naff, I may try another open source video editing software.