I think work hard or not does not relate to their body size, it really depends on their own personality. For me, I have my helper with us already 4.5 years, she is little bit fat but still ok ...hard work....Recall 4.55 years ago, I first time to hire the maid too, what we have considered were
1) not too small and short (at least they can help to handle some tough work and able to move some heavy stuff)
2) have experience to take care kids in the past (no matter in HK or not).
3) better they got married and have their own kids as it will reduce the risk they get pregnant in HK after you hire them