I believe it refers to the expansion of industry, economy, and society on a grand scale, and the resulting indulgent nature of people in such a situation. This word is also often applied to things which are luxurious and over the top; lavish.
The count had expansive tastes; only the best wine, women, and caviar were available at his lavish parties.
lastly, an 'expansive mansion' is quite literally very large. However, since you are talking about 'expansive days' then I would bet it refers to society either growing fast, being lavish, or both.
The implication is that Di Gamma Pi is a very large and nicely furnished fraternety house.