We have a dainty little one. She's a little bigger than yours, but at 20 months only weighs 22 lbs. Not SUPER tiny, but still pretty slim. She's always (from what I understand) been small and doesn't gain weight well. She wears 18 month dresses and shirts because she has long limbs, but pants are the hardest thing to find! Nothing stays up, so she wears 'capris'; that is, infant sized 6-9m pants that are inches too short.
I do know what you mean about being happy when they FINALLY give you a glimmer that they might begin doing something you've been working so hard to get them to do.
Keep up the good work.
With your mom, if she is always like this don't expect her to be excited in the same ways that you are, particularly about the things she is clueless about. Having her say that maybe your Dd would be 'fat by the time she is 12' could also be a way to suggest that the struggles toddlers have ARE often resolved over time. As maddening as it is, don't hold it against her. Is your mom supportive in other more tangible ways? Treasure THOSE things.