A bigger cockpit window would increase the pilot's vision of a runway. So why can't they be bigger?
Note that this question covers cabin window size already, but this question is about cockpit windows specifically.
The reason is that bigger windows really aren't necessary. They are plenty big for the crew to see the runway just fine. Pilots have more than 180 degrees of visibility, and plenty of vertical visibility as well. Traffic routes and patterns are designed to make it easy for the pilots to see the runway as well. If the runway is behind you, then you need to turn around. If it's below what you can see out the window, you are too high anyway. And if it's above what you can see out the window, you probably have more important issues than finding the runway.
Larger windows can help spot danger like other aircraft, but airliners are almost always flown under Instrument Flight Rules (IFR) where ATC is responsible for keeping them clear of conflicting traffic.
In the past, some airliners did have larger window area. This was more important in an era where instrument navigation was more limited, and the pilots relied much more on their view outside. It was also more common on aircraft with military heritage, where it was more likely for pilots to find themselves relying on visual references instead of navigational aids.
However, modern navigation systems have made larger windows even less useful. The instruments can tell pilots just about anything they need to know about anything they can't see out the windows. The 737 actually started with "eyebrow windows" above the main row of windows. Modern systems have made those less useful, and the extra holes in the fuselage can cause cracks over time. Most 737's delivered today don't have them, and some of the old planes even get those windows plugged.
Aside from the functional reasons for not increasing window size, the same structural concerns of cabin windows will apply here as well.