My mum has been doing the same thing to me! She's telling me that I.'m already too skinny even though I've been down to 95lbs in the past (when i started university) without a remark from her. She's been saying it since I hit 115 lbs.
This is despite my mum and my big sister being both 5ft and 90lbs and my younger sister being 4'10 and 80lbs. (I come from a teeny tiny small-boned family. I remember a doctor once weighing my sister and saying that she was underweight according to the charts BUT she looked perfectly healthy and all her bloodwork was fine).
I've totally experienced this, I'm smelling what you stepped in! My husband and best friend said that when I weighed 205 I carried my weight well and didn't look that heavy. I used to use that as an excuse, but just reached my limit after seeing my "scary number" on the scale. I let them know exactly what I wanted to do (get to 150) and how (diet and exercise) and what I need from them (love and support).
Sometimes it's best to get everything out in the open right away. Sometimes it's best just to smile and nod. It depends on how much patience you have to tolerate annoyances and deal with the danger that it could push you back to unhealthy habits. Stay strong, I'm sure you'll figure out what's best for you!
My mom once forced me to get on a scale in front of her because she thought I weighed under 100lbs. She since hasn't mentioned anything regarding my weight.
We both strength train so I explained to her that we're both probably heavier than we look (and consequently she weighs more than I would have guessed).
I think her and everyone else also realized that if I'm LIFTING REALLY HEAVY WEIGHTS that it would be quite difficult to do if I wasn't eating. It's not like I do this in secret...I'm the heaviest lifter in our weight training class. Everyone can see what I'm lifting.
I haven't exactly gotten the "you're done yet, right?", but I've had a couple people seem shocked that I want to lose 40 more pounds. That will put me just barely into a healthy BMI range. I'm just barely out of "obese". I think I'm going to just start saying I want to lose "a few more pounds", or "I'm not quite done yet".
I want to lose 40 more too, which will leave me slightly heavier than the "healthy" BMI range. (May adjust the goal as I get nearer.) Yet someone told me the other day that I don't need to lose any more. She might've just been trying to say that I look good now, but it didn't seem that way, more like she actually thought I was far lighter than I am. Kind of frustrating.