Curious to know who else has thought about, or committed to, downsizing their layout and collection.
To give you my personal experience, I started out with a multitude of scales. I was gifted G scale LGB sets as a kid, and enjoyed setting up big circles of track on the floor and going around and around. Once I got some straight track to make an oval I was set. I'd get a train going around at a decent clip, lay down inside the oval with a snack and a drink, and read a book about trains while watching my little 0-4-0 go whizzing past.
Got into HO scale when I got back into trains as a teen. Able to pack more realistic track configurations into the same space, more prototypical train lengths, and let's face it, bigger steam engines. College had me into N scale thanks to space and cash restrictions, making little switching layouts to do something other than stare at a textbook or computer screen.
Due to family history and a once-in-a-lifetime visit to the National Railway Museum in York, England, I decided I really wanted to go with an English theme. About a year ago I focused on UK OO scale and started putting together a small (18" x 48") switching layout. After that I realized that I wanted to really concentrate and refine what I had.
Sold the G scale, the HO, the N. Picked a time period and a location to base my little switching layout off of, and did my research on motive power for that time and place. Now I'm down to a single 0-6-0 diesel mechanical shunting engine and eight weathered freight cars. It's easy to keep the track clean and clean the wheels of my rolling stock. I do find I miss the loop to just let an engine run free from time to time, but overall it's a nice, compact, easily quantified collection where every piece gets used every time.