Korin made Ultra Senzu Beans cause they are faster to make and are twice as strong as a regular Senzu Beans. They are the size of a Base-ball and they come in different colors besides green. If a regular person eats more than half they will die(this doesn't apply to Saiyans).
Korin doesn't make them no more cause no one could handle them and kept on dying,so he stoped making them and he got the remaining beans and put them in a secret treasure chest under the Ocean.
But one day Majin Buu was swimming with Hercule and he saw something shiny under water so he went to go get it and it turned out to be a chest, when they both got back to shore they opened it and saw two large beans they both ate them. Buu was just fine after he ate one but Hercule died cause he ate the whole thing, luckly Buu brought him back to life.
And so the last Ultra Senzu Beans have been eaten and Korin doesn't plan to make any more anytime soon.