Whenever people say things like "they must keep you busy" I take it as a positive comment, because the person is giving recognition that being a mum is a challenging, time-consuming and important job. I'm sure that most people mean these things as a compliment, not a judgement on how many children you choose to have and how you decide to space them. But obviously lots of people find it rude, so I will be careful not to say this sort of thing in future.