There are a few ways you decrease the size.
The majority of storage space goes to either images or audio.
For images can either use the free app Xnconvert Photoshop or Krita to convert the image type. RPG maker supports .jpg, they require less space than .png which you only need if the image needs transparency so having the back ground images as .jpg is what I do.
You can also change the dimensions of the images, you only really need them to be the same as the resolution Xnconvert can also do this but it would be better if you used Photoshop or Krita for this.
Or you could change the compression in Photoshop or Krita while you're exporting the image.
Another method is to use WebP Converter for RPG Maker MV but the editor can't read WebP (but it can encrypt them). So this tool should be used for deployment.
As for the audio the only thing you can do is change the file type from .ogg to .opus which cost less space without decreasing the audio quality, you can use Opus Converter Tool for RPG Maker MV for that.
Decreasing audio quality also helps to decrease space but you'll need audacity for that.
You can also use MVStripper to get rid of unused files.
Here's links to the stuff I mentioned
https://www.xnview.com/en/xnconvert/
https://aceofaces.itch.io/rmmv-webpconverter
https://aceofaces.itch.io/rmmvopusconverter
https://forums.rpgmakerweb.com/index.php?threads/mvstripper-remove-unused-assets...
Be sure to check the details of each file before conversion so you know what you need to convert them into.
That's all I've got hope all this helps.