Decreasing the ratio of acrylamide:bisacrylamide when pouring an SDS-PAGE gel makes the gel "thicker" (less porous).  Can you explain why?

Sure, I'd be happy to explain.

The ratio of acrylamide to bisacrylamide in an SDS-PAGE gel determines the porosity of the gel. Acrylamide is the primary component of the gel and forms long, linear polymers. Bisacrylamide, on the other hand, acts as a cross-linker between these polymers.

When you decrease the ratio of acrylamide to bisacrylamide, you're essentially increasing the amount of bisacrylamide relative to acrylamide. This leads to more cross-linking between the acrylamide polymers, creating a denser, less porous gel structure.

In other words, the gel becomes "thicker" because there are more cross-links holding the structure together, leaving less space for proteins to move through during electrophoresis. This can be useful for separating smaller proteins, which might move too quickly through a more porous gel.