Wow, I gotta admit, this sounded sadly familiar when it first started but turned into quite the opposite!
First thing I have to do is congratulate you on valuing your wifes happiness above her weight. God knows how many guys would kill to have there significant other say something like that, and then how many more would secretly try to push them to that weight to make the dream come true.
As for the dilemma you face, I honestly don't know, Like I said, it is as left field as I have heard in this community and It sounds like there could be a large variety of motivations behind that single comment.
Part of me thinks maybe she issued it as a challange, to see you prove everything you have said to her, I mean if you are as serious as you tell her you are, I'm sure she would have expected you to react by encouraging her to reach that weight.
Then again, she could be expecting such a reaction so she can throw it in your face and say how selfish you are to want it to happen.
(lol women are so complex and scary)
Anyways, the best thing to probably do would be to have an honest and upfront conversation with her. Ask her about the comment, see if it was serious or off the cuff, and explain how you felt in reaction to it. Chances are what she says will be totally different to anything us randoms can come up with
I think it is incredibly sweet that despite your preferences, you reacted the way you did, it proves how importent to you your wife is and hopefully she will see that too.
Good luck with it all!