If you export your project as an .mp4 file and you require a smaller file, say because of down load time. You’ll have to use a utility like handbrake.
Pencil2d assumes that your bitmapped images and sound files are a fixed quality, defined by Pencil2D.
On a project that I have recently been working on, I saved the bitmapped files in indexed format, I.e. 256 colours and my sound files were saved in a low bit rate format.
These actions did not reduce the size of the .pclx or the .mp4 files. The only way to reduce the file size of the .mp4 file was to reduce the number of x & y pixels.
Handbrake with a little experimentation produced the desired results, with very little loss of either sound or picture quality.
Handbrake is available for McOS, Linux and Windows. The current Windows version says it’s for Windows 10 & 11.