While in pac-man you can see a very clear main loop, where there's a target of a whole bunch of jumps back, it's interesting to see that Q-Bert doesn't seem to have a main loop at all and seems to be written in an event-based style.
These are really interesting visualizations, wish I could see visualizations of modern 4K demos like this.
To visualize a modern 4k you'd probably have to strip off the UPX (or similar) executable packing and include symbolic names for all the external libraries they call. Otherwise, this approach could work pretty well. Having no external dependencies to speak of certainly makes old console games look tidy.
Combat code "pathetically simplistic" in this view?
Nah, it looks well-planned.
Combat, as the launch game for the 2600, was designed simultaneously with the hardware itself. It had some influence on the way the hardware design evolved; in a way, the 2600 hardware is kind of optimized for playing Combat. That might be one reason why it can fit into a 2K cartridge while a typical 2600 game uses 4K.
Obligatory link to the "definitive COMBAT disassembly" over on AtariAge.