Webm export estimated file size vs reality

Hi all,

I've a project that requires a webm export.

• uhd sequence (58secs)
• export settings - webm VP9 1600kbps 8-bit, metadata none

It's saying an extimated file size of 12mb which is roughly what I need for the web application. I'm getting 219mb (tried twice). I can trial and error to reduce the export size but the webm export takes about an hour to spit out my machine. It'll take a few tries to export and it's so timeconsuming considering the overall output size. 

Is it possible to make the estimated and actual export size more in line? 

PC - Intel(R) Core(TM) i7-6900K CPU @ 3.20GHz 3.20 GHz 64.0 GB RAM