I am a vegetarian. I don't have any complaints about the quality of airline meal. I am satisfied by what is offered and often my meal appears to be even better than what is served in the general economy class.
However, recently I flew back from Honolulu to Minneapolis. The vegetarian food was not only terrible, the quantitity would have been insufficient even for a two year old! It was barely a spoonful, concealed under much of aluminum foil. The irony was that the sticker said, "specially prepared for ..."

Have you ever been served an airline meal that was grossly inadequate in quantity? I am not talking about a short flight but one in which meals are served.

I am a vegan. I usually choose the non-dairy veggie option. The airlines rarely get it right even in biz class. I often get a garden salad thick with shredded cheese. It is easy enough for me to put aside the tabs of butter, the tub of cream cheese (with the bagel) and the carton of milk that comes with the breakfast cereal. I just wish they wouldn't waste it on me since I don't use those things.
I was once served a duck and some veggies in a broth on a flight from Hong Kong to Seattle. It was marked: "no meat meal".

I'm not a vegetarian by any means nor an expert on it, but what airline did you fly which you had that encounter with? Just curious.

I would ask for a double helping. Hopefully, they had other food to supplement..

I always order a vegetarian meal. Actually, I like to try different kinds: Indian vegetarian, Jain, vegan. They are always much better (to me!) than the regular meals.

One of my first times flying J class, the FA came up to me and apologized- he said he had never seen a special request meal with so little food. He said this to me before he served it. Then he apologized again as he served it. I forget the details- I think it was a breakfast meal, as I remember him offering boxes of cereal and a banana to "supplement" it. The thing is, it was fine for me. I am a big eater, so it might have been the excitement of finally flying J, as the FA did not think it was standard.

A few times after that (also in J), I would be served the first part of the meal- a salad and/or fruit, typically, and think (as I looked around at the "regular" meals that appeared to have a lot more food) "hey, is this all I get because I ordered a special meal?" Then I remembered, relieved, that the salad was just a starter. Most of my flying career has been in Y, where they serve just one "plate", until recently.

These have all been international flights. Once, on a connecting flight in the US (arriving from Y class international), I was in first class, and served a "regular" meal. I informed the FA I had ordered a vegetarian meal, and was told that that option was not available on that flight. I was surprised.

So, no, I have not had that experience of having an inadequate amount of food (although the FA thought so!), but I have worried about it!

It was Delta flight from HNL.
One good thing about special meals is that you get served first!
On a different note, when I was in Hing Kong 35 years back, the restaurant in the Hotel I stayed had never heard of vegetarians. They gathered whatever leaves they could lay their hands on, cooked it, and served me with butter. It is the thought that counts!

Hawaiian food will give it that exotic and sometimes weird exotic taste.

I work at a Japanese company, and for a while we had an Indian guy on our team doing database work. He was strictly vegetarian. We were out having drinks one night, and said guy came with us. A kind Japanese lady was trying to help him with the menu. Part of the conversation went like this:

Her: "What about this? It's duck."
Him: "Ummm... I'm vegetarian."
Her: "But duck is a bird!"