It's totally normal, I promise. Breastfed babies don't know how to overeat. You just obviously have a baby stocking up on food supplies at the moment.
My DS was breastfed and seriously put on several pounds a week for the first few months. He had about seven chins at one point. Medical professionals complimented me endlessly on the quality of my milk because he was so chubby that no-one could believe he was breastfed.
At 3mo, he was in 6-9mo clothing and by 6mo, he was wearing 12mo clothing. I too worried a bit that he was overeating. But actually, it came in very useful as he caught a few vomiting bugs last winter and lost weight quickly. Because he was a big baby, I didn't have to worry too much.
And when he started crawling at around 7-8mo, I noticed his weight gain slow down. Now he is 14mo old and running around, even though he is still an amazing eater (and breastfeeds at night), he is not at all chubby. All trousers are too big around te waist. In fact, now he can still fit into 9-12mo trousers he was wearing nearly a year ago!
So just feel proud you are doing a great job feeding, your baby is getting what they need and growing well. When your baby cluster feeds, this is a way of working out the exact milk supply they need by stimulating your let-down, not you overfeeding them.