I suspect that the market for bikes designed for very small people isn't large enough to attract manufacturers and that's probably why you won't find many ultra-light designs. Also, since the manufacturers of bikes can't control who rides them once sold, there could easily be problems of bikes for lightweights collapsing when being ridden by other people. There might be unpleasant accidents and court writs.

The Brompton is extremely versatile in the range of people it will fit. LittleYip will easily 'fit' it. In fact at 5'4" she isn't that small for a lady. I have a friend who is 5'1" and she bought one the bikes IKEA gave their workers a couple of Christmases ago. There was a flood of them on ebay.co.uk at the time. It fits her well and she is easily strong enough to carry it a short distance, even though it weighs in at about 13KG. That was very cheap, but has an ungainly fold in comparison to the Brompton or its clones.

Most fit women could carry a Brompton. Women are adapted by nature to carrying armfulls of babies, shopping farming and gardening tools, and are pretty strong in my experience. (ducks for cover).