For Interactive Templates builds, we need to apply some other settings (e.g. encoding) to the video assets to make sure it can be played/paused correctly on all devices. Some old devices or specific browsers might have issues handling the playback.
That is why the size becomes bigger after exporting.
For most ad networks that use inline HTML, the video clip is actually embedded in the source code, and this will also increase the build size. This rule applies to all inline HTML and is not really avoidable. All video/image/audio assets will have a slight increase in size.