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I believe the ZIP compression, unlike MOBI, is not random-access, that means that the contents must be uncompressed first before processing. It doesn't mean, however, that the full ZIP must be uncompressed, just the particular file (flow, chapter) that is being used, that's why some readers have size restrictions on this. Depending on the available memory, a given device might be able of storing the whole uncompressed book in memory, and so it wouldn't need to access the original epub file anymore, but other devices (or with larger books) may need to access again the compressed file every time you change flow (jump chapters, follow hyperlinks, etc.).
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