My BBL garment is really tight on my butt. Should I cut it to make hole at the butt way bigger?

Dear benedek87,your garment should be tight but not too tight to cause discomfort. But every plastic surgeon has his own postoperative protocol which he recommends to his patients. I would suggest you to discuss this with your plastic surgeon.Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery

This is purely surgeon preference. Some surgeons use "open" garments that allow the buttocks to be free from compression. Some surgeons use garments that cover the buttock but have minimal buttock compression, and some use garments that provide full compression after BBL. There is some thought that less compression may lead to better fat take, and I generally agree with that. I think it's best to follow your surgeon's instructions, but you can probably get a better answer from your doctor than the one you got so far.

Hello dear!!

Thanks for the question and provided information as well. 
In first place i will recommend you to follow your plastic surgeon instructions. If it is something you can handle i recommend you to buy another type of garment or buy a customize one that will fit perfect all your measures. Good luck.-