Stremio has a local cache to temporarily keep the video files you stream.
This has two benefits: first, it allows Stremio to pre-buffer the video ahead so as to ensure smoother streaming. Second, it keeps the videos in the local cache so you can continue watching them offline.
How much Stremio keeps depends on the Cache setting in the Settings panel. Stremio may keep up to the set amount (e.g. 2GB) on your disk, unless the currently streamed video is larger than that, in which case it has to allocate enough storage to keep it until you're streaming it.