Which all goes to prove a rather strange thing about memory. The more you have to learn the easier it is. The tune supports the words and vice versa, the use of imagery, mnemonic ideas such as starting and finishing rhymes also help. The fact is, narrative song is also easier to memorise than plainly lyrical stuff.
Another oddity is a thing called, 'working memory' there's a finite limit to what you can hold in memory in one session. There's a passage of time required in order to transfer material from 'working memory' to 'long term' memory. Its still a case of 'nothing in excess'.