Let things unfold naturally.
A conversation about it beforehand would likely feel very awkward for both of you. And keep in mind many women are just as insecure about themselves once their clothes come off and their lovers see it all: we might be wondering what he thinks of the extra lumps and bumps, the stretch marks, the size and colouring of various body parts, and so on.
My point is, most of us have our body issues. Think of how you feel about her, despite the extra weight she is currently carrying - she will probably feel the same way about you, despite your own concern over your size.