So..
I love the game, but I was pretty upset that bunnies(usagies) can only have flat chest. But I saw that someone modified their save file and achieved bunnies with big tits. However they didn't tell what values needs to be changed or they just changed multiple values and managed to somehow get what they wanted, so they just don't know exactly what to change and didn't bother going through all the changes and see what caused it. Luckily I'm pretty stubborn and all I saw that it was possible to have big tittied bunnies. So I downloaded the save file that had the changed usagi and I compared that with my unmodified bunny and using that I figured it out what needs to be changed.
So, I'm gonna try to tell what you need to do as simple as I can, so it's not confusing for anyone.(I'm pretty new to changing save files, so if I could do it,you can too :v)
First, grab your save file and upload it to SaveEditOnline. After that you need to find an usagi ID, to do that first search "RaceTraitList". You gonna see a bunch of text but only specific part of it matters here. First the IDs, those are the ones after "m-UnitList", the second ones are the green numbers at the end of every line. The green ones are the ones that we need to look at first, as you can see every ID has 2 lines with it and we gonna have to look at them in pairs because of it. So, finding an usagi one, to do that we need the first green number to be 63 and the second 64 (those should be usagies every single time, unless you changed them yourself prior to this). Once you found one, go and look at its ID number and remember it.
Optional: you can do it with the unique usagi(Vivi) too, but for that look for 63 first and 75 second green values.
Second big step, search for the ID this time (like this: "m_UnitList > ID >" ,the ID is whatever number your usagi has).
Lets start with the normal usagi ones: hit ctrl+f and search for "tits" ,you gonna see titstype and a value next to it (probaly 0), here you can decide how big you want the titties to be (0=flat, 1=small, 2=medium, 3=big, 4=huge). You gonna need to do that 2 more times, just follow the highlighted search results. The website might split the information about the usagi into 2 or more pages, so you might need to find the 3rd one on those, but as long as you changed the value 3 times you good to go.
And thats it, you can download the changed save file and paste to it's folder, your usagi gonna have big tits and it's offsprings too(except in loli form), just give them milk, you can still customise them with cosmetic potions, but be careful because if you change the tits size you can't revert it back, that's why you need to decide how big you want the tits to be during the editing (or you can just do it 4 times with 4 different tits size for 4 different usagies and keep them safe and use them to make offspring and edit those instead, so even if you mess it up and you don't have a relative recent save you can just breed the safe one again, instead of going through all of this again xD)
If you did it with Vivi, her offspring gonna have big tits, but she won't have visual changes but her stats gonna say she has big(whatever size you chose from 1-4) tits.
Notes:
About lolies, you probably could change those to have big tits too, but personally the small furries(hitsujis and usagies) are loli enough for me, so I can't be bothered to change different values and see if it's possible or not.(You would probably need to change every single one individually though, that's why I don't want to do it, because what we did now, you only need to do it once and then you can just breed them to get more)
Same with Vivi too, I tried to keep her outfit and hairstyle with big tits, but I couldn't, she lost those whenever I gave her visual big tits.
I did this with 1.2.7.0 game version, can't guarantee if it works for earlier or later versions.(the changed bunny that I was talking about was from an earlier version too, so I had doubts if it's possible or not when I started, but luckily it still is)
Always make backups of your saves.
If you have a bouch of slaves/monsters, SaveEditOnline might not load any usagies even if you have, so either delete some or start a new game.(I did this while I had 19 slaves/monsters, so if you don't want to delete that many, it's a safe number)
Feel free to ask questions about this, I'm gonna do my best to answer them here.
I also wanna thank everyone that posted information about save files and values prior to this, I couldn't have done it without those.
Enjoy your big tittied bunnies