Well, I'm a 'BBW' lesbian, but I'd personally not want to date a woman who identifies as a BBW admirer, because, to be perfectly honest, the whole BBW thing feels like fetishisation of larger women. I've seen the BBW label applied to women who are certainly large but don't have attractive faces or attractively feminine figures, despite the second 'B' supposedly meaning "beautiful".
I can think of things besides my above-average size I'd rather a lover like me for, like my freckles, my geek streak, my dubious artistic abilities.
In an attempt to be helpful, I'll give what you might be doing wrong a go. Thin to average women are usually self-conscious about their figures, but have enough self esteem in their physical appearance that if a man says they have a nice figure, they might not believe him, but they'll believe he believes it. A fat woman is not just self-conscious, she's constantly being told her size is a) completely voluntary and b) disgusting. Regardless of how attractively that fat is arranged, a fat woman living in modern society has extreme difficulty believing somebody might find her fat attractive.
So, start with physical complements that would be just as true if she were skinny: nice eyes, hair, freckles, whatever. As soon as you know her well enough to complement her on her personality, do so. Don't mention that you admire her figure until she's comfortable with your total admiration, which if she's been very badly broken by anti-fat messages, honestly may not be until you've been married several years. Obviously, if you have sex before she's really comfortable with your admiration, you'll have to mention that you don't find her figure off-putting, but not off-putting and attractive are two very different things.