Basically, Catholics have an oral tradition and a magisterium as well as the written text of the Bible. With the Jews it was the same. You have the Hebrew Bible, and you have the revered history of oral interpretation of the written text.
As Scott Hahn has pointed out, nowhere in the Bible does it say "sola scriptura." But there are several places where it says something to the effect of "these things I have written, but other things were spoken as well." I think especially of the last verse of the Gospel of John: "Jesus did many other things as well. If every one of them were written down, I suppose that even the whole world would not have room for the books that would be written." Catholics believe that some of those other things, not mentioned in the Bible, may have been passed down orally in tradition, and that they do not contradict what is written. The job of the Magisterium is to accept, deny, or leave to private judgment things not directly written in the Bible. As I said, both the Immaculate Conception and the Assumption are ideas going back to a fairly early period of the Church, though only fully defined at a later date.
You are right...Jesus did do other things not noted in the scriptures. However, orthodox christians do not make these extra-Biblical and legendary events into doctrine as is done with Mary. To attribute to Mary an Assumption into Heaven which is not stated in scripture but only legend is going beyond what is done even with Christ.