To clarify, the Defold Editor is written in Clojure, whereas the runtime component is written in a carefully selected subset of C++ in order to keep executable sizes small and compile times fast.

You can write game-specific extensions to the runtime in C++ if your game requires it, but most games are authored in just Lua.

Edit: Typo.

Defold is optimized to produce small binaries, and features you don't use can be turned to to further decrease your bundle size.

It also focuses a lot on providing fast iteration times. You can hot-reload changed assets onto a device while the game is running. For most asset types, you'll see the change on device in less than a second. Similarly, a Build and Run cycle from scratch for a moderately complex game project such as Blossom Blast typically takes around a minute or so, and subsequent cached builds can start in a couple of seconds.

(Edit: Full disclosure, I worked on the Defold editor.)