http://www.whathifi.com/blog/the-cd-is-30-years-old-today

CD facts:

Why the 12cm disc?
No doubt influenced by both the conductor and his friend Ohga, the CD format changed again, growing to 12cm. Why? Simply because Sony insisted that the running time should be sufficient to hold the whole of Beethoven's 9th Symphony.

And given that the longest-running version of the 9th in the Philips-owned Polygram catalogue – Furtwangler's 1951 Bayreuth Festival recording – was 74 minutes, that became the standard.