Chemistry: You’re either asking this because you have doubts about why you’re attracted to an overweight person, or you’re asking because you attraction to your partner is weak. If it’s because your attraction is weak (and if you’ve gone on more than a couple dates), then your chemistry with your partner is too low to make it worth continuing. Do yourself and your partner a favor and leave, rather than dragging it out. On the other hand, if you’re asking this question because you have some doubts, then consider the character implications.
Character: With a lot of these questions, it’s not the “what” that makes a difference, but the “why”. Why is your partner overweight? Are they overweight because they are irresponsible? Or do they take reasonable measures to stay healthy? In the United States, even if you’re a responsible person who has a reasonable amount of knowledge about health and nutrition, it’s still very easy to become overweight. This is due to the abundance of food options, aggressive marketing from fast food chains and an over-reliance on eating out. So if your partner has the other hallmarks of responsibility (stable employment, organized life, good habits), then you shouldn’t hold their weight problems against them.
There is also an aspect of respect. Some people are overweight because they don’t respect their bodies. They may still lead responsible lives, but they could have a careless attitude when it comes to exercise or what they consume. This could extend to disrespecting you or being apathetic about things that matter, like choosing where to live, planning trips, or child-rearing.