Who enforces 8MB: MMS on most carriers tops out between 1MB and 3.5MB, so 8MB is the practical floor for anything watchable rather than a platform rule. It is also the size worth aiming for on slow or metered mobile data, and for WhatsApp Status over a weak signal.
What fits in 8MB: roughly 25 seconds of 1080p, about a minute of 720p, two 12MP photos, or a 100-page text PDF.
Worked example: a 40-second 1080p clip off a phone is usually 90-120MB. Targeting 8MB drops the bitrate to about 1.6 Mbps — soft on fast motion, fine for a talking head or a static scene.
Not sure 8MB is the right target? The hub explains the limits and the document method.