How to respond to someone who is being well, "nosy"

I live in a warmer climate and rent an apartment. The people here are not "friends" but they are neighbors. They are people who mutually rent apartments at the same apartment complex. I am sure they feel the same way about me and others. I am polite to them and would help them if they ever needed it. I have walked someone's dog when she was away for a couple of days and took the dog out for bathroom breaks and fed her dog. Things like that. But we don't hang out, don't socialize, don't exchange gifts and so on. On nice days the neighbors sit in front of the apartment buildings in small groups (maybe 5 people) and enjoy the weather. One of the people asks me kind of personal questions alot as I am leaving the building or entering the building. She doe not live in this apartment building but comes over to sit out in front of it with someone who does. For example, when I purchased a new used vehicle she asked a personal question about it in front of the group she was with. I was taken back. She will say things like "were you just at the gym"? Etc. She asked a question about loved one who was dying and I felt extremely uncomfortable about since I don't know her really. I don't even know her last name. Recently in front of the small group she sits with she said "Wow, you have lost alot of weight" and then asked how I did it. I felt very uncomfortable and don't even feel I owe someone an answer to a question just because they ask. Instead I had things in my hands that I was carrying in and I said "Let me get these packages inside". What is a polite answer to give people who ask personal questions that you are not friends with? I want to maintain my personal boundaries without being rude but asking someone about their weight in front of a group of people is not polite either.

I probably wouldn't have the nerve, but I would want to say," I don't discuss my weight, but we can discuss yours if you like" Then everyone is now looking at her.

That sounds pretty harsh, but if the person can afford to do that then I agree with you, you have to know how to put someone in their place.

I would say I quit eating pizza and candy and did more exercise. Probably the truth.

A polite response could be "Thank you for asking, but I prefer not to discuss personal matters with acquaintances