At the risk of echoing the other women on here, yeah, a sit down is in order and with that I'd say to show some of your sense of vulnerability and that you're welcome to suggestions. Maybe she just doesn't know how to be a MOH? I know my MOH has a ton of things going on and doesn't feel comfy w/her weight right now either but I figure, thankfully a lot of this planning fun is taken care of since it's a DW and there is a coordinator at our wedding AI resort that will handle a lot (as long as we make the decisions, eep) so at this point, I am just hoping for emotional support from my MOH and for her to be by my side on the big day (and hoping for some support before hand to). It can be overwhelming to think of all the things that you have to do for a wedding but maybe sitting down and getting a list together and going over it with your MOH and asking if she can help w/some of it (maybe let her pick a few things that she thinks she can handle?).
I will say, I envy that you have a good deal of time ahead of you before the wedding and you're already this organized w/lists of things to do so in the meantime, breathe, step back and take a look at the whole picture and try to enjoy these early planning stages. I can only assume that when your day comes, it's going to be very well organized and you'll be totally on top of it.
Suggestion w/the dress shopping w/your MOH, if she's feeling insecure, throw her some really solid, sincere compliments while trying on dresses or maybe suggesting a style that brings out her best body structure features.
Good Luck!