I honestly think if I could be lighter I would have an almost perfect life
In which case you must have lots of lovely things to talk to people you haven't seen in a couple of years about!
Seriously, I think this might work! Yes, get the "Helloooo @Trumpstoupee, you look well"'s out of the way and respond with; "Thanks - I am really well - it's been the most wonderful couple of years/ few months/ time of my life - so much has been happening I hardly know where to begin! Isn't that right DH?!..."
Then onto:
Updates on your DC (if you have them) - schooling/University/ Sporting/ Arts achievements /their wonderful new partner/ recent engagement/ upcoming wedding/ the 1st Grandchild they're expecting, stories of amazing places you've visited on holidays, tales of your and DH's work successes, the 'Forever Home' you've always dreamed of owning and have just moved into (or have recently been to view perhaps?) or exciting hobbies you've taken up?
Have a good think about the impression you want to leave people with - it sounds like you simply want to make sure that you leave them with the focus on you as a person, rather than just thinking that you've put weight on. Ensure that you have lots of positive, engaging, uplifting news and stories to share and the next time they think of you, it will be of how much you're getting out of life - not how much cake you're eating!
If you have plenty to talk about ready to go, you should easily be able to avoid having to fill an awkward silence with chattering about your weight - please try not to mention it, as this just opens the door for the other person to pass comment or ask questions about it, which rather defeats the object.
I am hopeful that as well as leaving others with a lovely impression of how much you're enjoying life, using this 'method' will also leave you feeling more upbeat, as you'll be focusing on all the wonderful positive aspects of your world - which can be easy to lose sight of when you have an issue (such as a large-ish weight gain) that can be so all-consuming. It might also help to remember that lots of other people at the event will have the same or similar anxieties over various things that they would prefer not to focus on either.
A final piece of advice I was given when I was Chairing a Public Speaking Team in my mid-teens, was to 'Never announce your fear'. In other words, don't get up in front of an audience and start off with; "I'm sorry about this, I'm really nervous - you can tell, can't you?" because the whole audience is then just sitting there, waiting for you to make a mistake - you're expecting the worst from yourself, you've told them to expect you're not going to be any good and with all that negativity and pressure, the likelihood is that you won't be. You'll forget something, or your mind will go blank and your self-fulfilling prophecy will have come true! Or in your case, don't start off conversations by drawing attention to your weight, or bringing it up before they do because if you do, then your weight will become their focus and when the party is over and everyone is going about their normal lives once again, a week later when that person brings you to mind, the first thing they will think of will be your weight increase. How it has clearly badly affected you in a very negative way emotionally as well as psychically and ultimately they will probably be left with a lasting impression of pitying you over it. You can do much better than that - there are SO many wonderful things going on in your (almost perfect!) life, that you are going to create your own, positive narrative around yourself and ensure that when they think of you in the weeks to come, it will be fondly, happily, glad that you are content and perhaps even with ever-such-a-tiny-slither of envy... 
I sincerely hope that this is helpful to you OP, wishing you the best of luck - I hope you enjoy the gathering and a positive boost to your MH! 