Yahoo mail to roll out unlimited storage to all yahoomail users, that means there is no barrier on storage space you get. Yahoo is trying hard to stop the dominance of gmail and to increase the user base. Few months back they Ajaxified the yahoo mail and now they are offering unlimited storage.

John Kremer on Yodel blog

As promised, we’ve started to roll out unlimited email storage to Yahoo! Mail users worldwide today. When it hits your account, you’ll notice the storage meter has disappeared � meaning, you just don’t have to worry about deleting old messages ever again!

I would say its all started with Gmail features and their storage space, yahoo is trying to catch this. If i remember well a few years back when i first created my yahoo mail the storage space is just 4MB.