Starting with a processor box the size of a large matchbox mounted to my chest, with wires going to my ears into hard plastic ear moulds. Moving to all the electronics (including the ‘receiver’, the speaker element) going behind my ear with a tube going into an acrylic or silicone ear mould. Finally today, it’s Receiver-in-Canal where the electronics are still behind the ear (but much smaller and direct Bluetooth connectivity) and the receiver moved to inside my ear canal inside a tiny plastic tip, making it nearly invisible (invisibility is both good, and bad. It can be useful for people to know you’re deaf with a quick glance at your ears.)
There has also been a shift from analog to digital along the way. Interestingly there’s a product called Phonak Lyric that uses analog but in a new way (a product that sits right next to your eardrum for months at a time). I’ve heard good things about it for mild hearing loss.