Looking at one of the labeled diagrams [0], it looks like all the large areas of polysilicon are either serpentine structures that are probably resistors, or they're adjacent to one of the power rails. The ones you're referring to seem to be at the top of the instruction register section and right below the Vdd (-9V) line. So we may simply be looking at larger transistors to handle more current and voltage.

[0] https://lh3.googleusercontent.com/-nHUr8PLBS-8/WF4ffgi2aiI/A...