mp4 file expands wildly when shortening clip
Not sure what's happening, but, when you export your edited film, keep an eye on the data rate. That's really how you keep files small. The lower the data rate is, the lesser the quality of the video image though.

So, when you export:


For a 720p final file, make sure your data rate is no smaller than 5
Mbps.
For a 1080p final file, make sure your data rate is no smaller than
10 Mbps.
Codec: H264.
Field order: progressive usually (depending on your raw footage).
Aspect: square pixel usually (depending on your raw footage).
Audio: AAC, 4100Hz, Maximum Bitrate.


Note: I'm not familiar with Lightworks' settings, so some of the options mentioned above might not be available to you when you set up your export.

Let us know if this keeps your file small.