I have the same problem
with the old version of shotcut the size exported was similar to original video.
in some cases smaller (with quality 100% of the origilan)
i exported in mp4 with “codec” mpeg4 and quality 100%
after updating i’ve now the version 18.11.13
the wideo exported in totaly back with audio.
Many preetting dosen’t work, balck scren with audio too.
Presert youtube work, i try to use it (ALL setting right, and 100% quality as ever), preset are lbx264 codec but the size of file are 4X biggher than origilan file.
last export video was 1.95GB with long of 16minuted and 16sec
after exported is 10.5GB with long of 10minutes and 40 sec.
(i normally expetted to have a file of 1 1-5GB or smaller)
sometingh anomalus appened in shotcut.
why this terrible changing?