Alright, I'm going to tell you what I know based on what I've experienced with my girlfriend.
Most girls are sensitive about their weight. Even if you say "I LOVE YOUR BODY, it's so sexy, I love that you're chubby/fat/curvy." Even though your explicitly saying you love her extra weight, chances are her woman-brain has interpreted that as you calling her fat. And if she doesn't want to be "fat," then you're getting into trouble.
Every spring my girlfriend starts running like a madwoman and loses weight all through the summer. She's 5-feet tall, probably around 200 pounds in the winter, which I love. In the summer she might drop to 180-175lbs. She's still beautiful but I do prefer the fluffier winter-time version.
I once made the mistake of asking her why she's working out so much, I said your body already drives me wild there's no need to loose weight! And she got pissed. Because it's her body and her health and her choice to loose weight if she wants to. And if I want her to be overweight simply for my viewing pleasure, then I'm no better than a man who wants his girlfriend to lose weight for his viewing pleasure.
And yanno... She's right. I love her for more than just her body. So if she's not the perfectly plump BBW pornstar I love watching on xhamster. That's ok.
Girls do not like the idea that her body, fat or thin, is a majority of the reason you're attracted to them. If we're all being honest, men are very visually aroused, so of course her body is a major part in your attraction to her. But girls don't see it that way. They're big on the romance and inside beauty stuff. And it generally won't end well if you try to make her see it your way. Whatever her size, it's wise to play it off like that is only 1/10 of why you love her.
And although it may be counter intuitive that's my strategy for keeping her plump - positive reinforcement.
So I don't say anything about her exercise, in fact I'll be nice about it. If she tells me she ran three miles today, I'll say *great job!" Because weight loss or not, exercise is good and puts her in a better mood. I even bought her new running shoes for her birthday.
But If she ever mentions in passing something like "ugh, I can't wait to start running again. I'm gonna lose weight and get sexy for you." I react with mild offense and say something like "Crazy girl, you're sexy all the time." If she ever says anything disparaging about her body, I'll remind her that in my eyes things look the exact opposite. Then the next day I might be extra affectionate and kissy and tell again how beautiful she is and how much I love her. Her love-handles are her least favorite part about her body, they are my absolute favorite, I kiss them, and her belly, a lot during foreplay.
Like you, I don't necessarily want her to gain any weight but I do prefer when she's got a little extra on. My goal is to make her feel more comfortable with being plump. And slowly but surely, I think it's working.
I hope this made sense because this may be the only honest advice I've ever been able to give.
TL;DR: Make it clear that you love her and her body. Don't discourage her from loosing weight if she wants to, don't tell her to gain weight either. Most people in comfortable, loving relationships tend to gain weight anyway.