I can only give generic advise since you didn't not say what tool you use.
When converting to videos using the WEBM-container make sure you do not use the outdated VP8 video-encoding. Use VP9 instead. That's currently the best compressing video supported by Ren'Py and it's free.
For a game where you have no size restrictions (like max. 4MB to upload to a website) you probably want to use "constant quality"-encoding. In this case you control the quality vs filesize via the CRF-value.
Details here:
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Most free tools/converters are based on FFmpeg anyway and they disguise these kind of controls behind some quality sliders to make it simple for 'casual' users. That's okay and there is nothing wrong with that ... but to have full control over what's going on these tools are bad. That's why I personally would always recommend using FFmpeg directly on the command-line.
I believe
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had no support for WEBM. But don't quote me on that.
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does support WEBM, but that's an editor and possibly a little overkill for just converting a video:
For Shotcut the previously mentioned "constant quality"-encoding is listed here as "Quality-based VBR". VBR means variable bitrate.
Many "cheap" (Online converters are garbage. Avoid!) converters use an encoding-mode with a fixed bitrate. That's especially bad on videos with a lot of motion. The bitrate would have a spike on a scene-change but the encoder struggles to include all the data with the limited bitrate.
