Maybe you should nicely inform your mother that you are, in fact, eating a larger amount of bulk than most people AND you are eating more often. Her observations are correct.
You stated that you were eating, for example, salad and beans and your mother asked, "Are you eating because you are hungry"?
Well, if you are really eating things such as salad and other vegetables all the time, then it only makes sense that you would need to eat a lot and more often.
I think 2 cups of lettuce has 16 calories. The lettuce will initially fill up your stomach, but then, b/c it is made up mostly of water, it will quickly break down and you will be hungry again.
I would say you may be able to easily reconcile with your mother by helping her understand the science behind eating those types of food.
One other thing about the comments such as "Are you REALLY hungry or are you simply eating just for the sake of eating" (a comment which I have also heard from my mother when I lived at home years back).
I have a story. I recently moved to Germany and in the particular place I am located, every person who wants a dog is HIGHLY recommended (if not forced) to spend a few weeks with the dog in official Doggie School.
Wow, what a difference this training makes! Dogs are AMAZINGLY well-behaved here. Not just one or two, ALL of them (and there are tons of dogs here). For example, very few people need to leash their dogs - not even when in a park with many other dogs out and about. You will also often see people riding their bikes through town and the dog faithfully always running alongside. Keep in mind, this is a busy city.
So, what does this have to do with your mother's comment? I often feel that the world could be a better place if some parents just had a week of "Parent School" (And if I were a parent, I would go too).
If they could just understand the depth behind saying things such as "Are you REALLY hungry or just eating"?, then they wouldn't say it. Saying that is entirely counterproductive to ANYTHING positive that could have been meant in the first place. First of all, what exactly could your mother's intentions be? Well, if she has seen you yo-yo dieting, then she is potentially trying to 'open your eyes' to the fact that you could gain weight with such behavior. She is trying to save you from future pain possibly.
Her intention is honest enough. So, what effect does such a comment really have? Does it bring about anything positive? Not at all. In reality, this tiny comment infringes on your free will- your ability to make your own good choices is entirely negated. This causes anger, frustration, and resentment.
And I feel comments such as these could be easily avoided if parent's just had a "Parent School" to attend for a few weeks

A place to clue them in on the negative consequences a few, simple words could have.