Dezl 770: Settings > Map & Vechicle > Map Detail > More
This way you'll get the max detail of any map. After that, the available detail at various zoom levels depends on the map design and the device.
For example nuvi's 3490 "More" setting shows all detail of CN Europe NTU up to 800m zoom level (StreetPilot 1.2 Km) and "Normal" shows all detail up to 300m (StreetPilot 300m).
Gpsmap 620: Ther is no option for Auto Zoom ON/OFF and you can't disable off-route auto recalculation.