Re: ELI5 please
*... does it just keep growing in size forever?"
AIUI, this is why it is called block chain and not just chain. The miners compete to be first to generate a sufficiently large partial hash collision, which is a "proof of work" (because the only way to generate such collisions is trial and error). Once that is done the completed block can be promulgated amongst the peers, and its totals can be trusted, without any need to open it up and iterate through the individual transactions within it. You therefore only have to total the subtotals from the blocks which went before.
I visualise it like a long ledger in several volumes where each complete volume has been signed off. However as I'm the AC who asked for the ELI5 this is a highly non-authoritative answer!