ANOTHER BONUS: Quick Tips & Resources for BBW VRChat Avatar Creation (which isn’t part of the original post because of the character limit, whoops)
Like with Blender above, this assumes you already know the basics of Unity and VRChat avatar creation. If not, there’s a wide variety of tutorials for both online, most of which are high-quality and cover everything needed.
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Export your avatar models as .fbx files to preserve blendshapes without any issues. CATS can help you with this, as well as fixing improper humanoid skeleton assemblies/hip-to-pelvis angles that VRChat will have issues with, combining redundant materials and other useful things
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You’ll most likely need to reconfigure your armature’s non-humanoid bones for use with dynamic bone physics before exporting it. Try to set things up so that all the parts you want to jiggle later will have bones assigned and parented properly, and that they’re left unconnected to other bones at the ends unless it’s to an also-free-floating base bone
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If you want configurable weight gain, breast/belly size and other aspects, create animations using your blendshapes as properties, with 0 as the start and your max value as the endpoint, then assign these to radial slider menus. Make sure to animate using a copy of your avatar and not the original, Unity loves to stick them in a default animation pose and break everything, the saved animations will still work with the original however
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Particle effects can add quite a bit of enjoyable detail, from sweat and visible breath to lactation and other dribbling liquids. The Particle Ingredient Pack asset from the Unity asset store is free and incredibly customizable, with a bunch of prefabs that you can tweak to insane degrees via the Shuriken particle framework included in Unity. Parent your particle spawners to bones so they’ll follow your body with proper placement, and use world simulation and collisions as needed for desired behavior
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Sounds greatly enhance the experience too, both in an immersive sense and in a sensual one too. Freesound.org has a huge library of good free-to-use sound effects, among which are belly gurgles, gulping sounds, stretching noises, female burps, skin-slapping sounds and liquid sloshing, plus more
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VRC Avatar Actions helps immensely with configuring things, auto-generating animation assets, adding sounds/material swaps/object toggles, and a lot more, I highly recommend it given the pile of code and variable spaghetti that doing it all manually can become
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VRCMods has a wide variety of free Unity packages intended for VRChat use that can help you out, such as shaders and animations, as well as some premade avatars to tinker with
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If you need models to practice with, or even outright make fat avatars from, you can find plenty of options on Models Resource (game rips/work-safe models), deviantArt (original stuff/modified models and fan works/ a few NSFW models) and SmutBase (nothing but NSFW models/props/tools)
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In order to have your avatar’s fat actually jiggle rather than being static, you’ll need to have the Dynamic Bone asset imported into your Unity project, at least until the VRC team finishes making and finally releases their supposed better-performing alternative. Make sure to thoroughly test the settings for the component so you can get the desired physics via tweaking, and use colliders to have dynamic parts touch one another and not clip through each other (as much)
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Don’t let it throw you when VRChat starts complaining about your avatar in the SDK, and labels it “Poor” or “Very Poor” in performance. Any model that fattens nicely will have higher-than-usual polycount, most of the fun features like dynamic bones tank the performance rating very easily, and in the grand scheme of things, the vast majority of people ingame couldn’t care less about the performance ratings on avatars, strutting around in endless numbers of “Very Poor” ones with no worries. As long as the SDK will let you build and test/publish your avatar, which it should unless there’s a critical error of some sort, it’s all good. Aside from all that, I assume you probably won’t be busting out your fat avatars in a public lobby where there’s enough people for performance to matter, although if you do then all the more power to you for being so open.