The Meaning of Life, the Universe, and Everything.
In real life we breed bigger chickens and cows that give us more meat. That gave me an idea- mobs that have more drops. (Either with slightly larger models or not.) Basically the idea is that you can breed animals by crossing their food with an item (maybe an emerald, or maybe a mob specific food.) You feed it to the two parents and the offspring will now give twice as much of whatever drops they have (food, leather, feathers, XP). Say you start with two regular chickens and give them both the super food. Now they breed together and have a baby. That baby is a Level 2 mob, with double drops. Breed two Level 2 mobs up (or, just use the level two chicken's eggs... that makes chickens the easiest to do). Breed your level two mobs together. Being level 2 they take twice as much food as normal (they keep track and don't enter breeding mode until they get the last piece of their food to be able to breed). Give them the normal 2x food and they breed a Level 2 villager, but breed them with the special food again (twice) and they will produce a Level 3 mob that has 4x as many drops. The level three mobs can be bred by giving them 4x as much food as normal, or by feeding them 4x the normal food at the same time as some of their special food and they become Level 4 mobs with 8x as many drops.
L1 Mob- Normal amount of drops
L2 Mob- x2 amount of drops, takes x2 as much food to breed
L3 Mob- x4 amount of drops, takes x4 as much to breed
... all the way up to L7 Mobs that drop x64 and takes as much to breed as 64 mobs.
There are a couple mods that create feed bags. Creating feed bags to hold the correct amount might be useful. If you breed a higher level mob with a lower level mob you get a lower level mob (so you can't cheat your way to a lot of high level mobs.) Chickens, however, will always drop eggs equal to their own level... chickens are easy to breed in the game anyway.
The idea is that instead of having a chicken farm with dozens of animals you can have just a couple that produce a lot more drops. There are a couple mods out there that increase crop yields. This would do it for your animals. Each level could have a slightly larger model, but if you double the dimensions of a 3D object you increase its volume by x8, so even if you scaled them up 'max' a Level 7 chicken would only be twice as tall as normal. That still could be sort of strange though, and I suspect IRL that you can get animals with more usable meat relative to their size, so maybe just making them a tiny bit bigger would work.
The big payoff is that you could have really high yield farms without having to have tons of mobs loaded and causing lag. The cost per unit of food is the same (since HP don't increase at the same rate it would save you a little durability on your sword if you are killing them manually, but that is such a small benefit that it barely is worth mentioning... usually if you have mending the Xp from mending keeps your sword in working order anyway.) Maybe each level mob could have one more heart... that way it's a little harder to accidentally kill one. The cost of the special food to breed them up depends entirely on whether people think of this as a mod to simplify your farm or as a way to fight lag. It really could simplify things a bit- you could lead a super animal around on a lead much easier than 64 individual mobs, but other than that the extra food cost exactly matches the normal food cost for the yield and on a server this could make farms much less laggy.
Maybe there could be a config option to set a maximum level. Obviously, if you create a mob that has x1024 drops per mob just slaughtering that mob could cause a lag spike, but some people might find x64 drops a little OP too, and only maybe want x4 drop mobs.