A few decades ago Oxford Dictionary said it'd take 500mb to store the whole dictionary digitally. But today I downloaded a copy for my Kindle and it was less than 20mb. What gives?

In the early days of going digital Oxford said a long time ago that it'd take 120 years for someone to manually enter all the definitions, 60 years to proofread it, and it'd take up a staggering 500mb in storage space (or as much as 350 floppy disks!). But I just downloaded the Kindle version of the Oxford Dictionary and it was less than 20mb. So what gives, did we learn how to compress text 25 times smaller since then or what?