As I recall one was Maimonides, but he wasn’t alone, just the most prominent. He was 12th century, but I vaguely recall there was someone in the late 9th as well.
The one I had in mind who said millions of years came up with that because he concluded that if a thousand years was an “age” for humans, then it would be a thousand thousand for God, and since God is called “the ancient of days” then the universe must have been around for at least a few ages. It’s a strange sort of thinking to us, but useful to now if only to illustrate how much our understandings of scripture are worldview-dependent, plus how ludicrous it is to assert that everyone must have read Genesis the way the YECists do.
The idea of the universe being tiny then rapidly huge comes from an approach to passages where the first word is broken down by letter and the letters have their own meanings. It’s something I never really got into but has been perfectly acceptable during several periods of history. As I recall it relies on the shape of the first letter, which when properly done has a very tiny fine point on the right then expands almost immediately to be much larger – ב – read of course from right to left. The letter’s name means “house” in at least four ancient languages, for example Akkadian, besides Hebrew. So the meaning is that God was making a house for Himself that started tiny and grew to be vast. The second letter – ר – means “head”, and the third – א – indicates a beginning, so taken together it’s God’s house where God is the head of the beginning.
A book where I came across some of this again recently is In the Beginning . . . We Misunderstood.