File size is bitrate times duration: there is no other lever. Flippy converts your 8 MB budget into kilobits, takes 6% off the top for container overhead, hands 96 kbps to the audio, and gives the video stream every kilobit that remains.
An 8 MB budget buys roughly one minute of decent-looking 720p video. A 30-second clip at 8 MB looks genuinely good; past three minutes the math forces visible blockiness, and no tool can dodge it.
The fix for a long clip is length, not settings: trim to the part that matters, then compress. Sixty good-looking seconds beat five minutes of mush every time.