In the interests of credibility there's a limit to how non-magically strong an 80lb human can be. The following is a suggested table of absolute minimum character weight by STR, since lean muscle mass has a pretty consistent weight/power ratio. This should work fine for manlike humanoids, it may not work for halflings or gnomes (it wouldn't work for chimpanzees either). If a generated character has weight below these minima, simply raise their weight to the minimum given. Figures are derived from the PHB Encumbrance by STR table. Table can be ignored for magically augmented STR (eg Wishes).
STR Minimum weight
24-25 350 lb
22-23 270 lb
20-21 200 lb
18-19 150 lb
16-17 115 lb
14-15 88 lb
12-13 65 lb
10-11 50 lb
8-9 40 lb
6-7 30 lb
4-5 20 lb
2-3 10 lb
STR Minimum weight
24-25 350 lb
22-23 270 lb
20-21 200 lb
18-19 150 lb
16-17 115 lb
14-15 88 lb
12-13 65 lb
10-11 50 lb
8-9 40 lb
6-7 30 lb
4-5 20 lb
2-3 10 lb