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I think, like many things, it's what you're used to.
If you want to be able to switch mouthpiece size you need to develop that ability and those muscles to keep them in shape.
If you stick with one size, going back can be a rude awakening to muscles you haven't used in a while...

It appears to me that accidentally playing your old mouthpiece affirmed your wise decision to change it.

Sell the old one for a duplicate new one and you won't make that mistake again!!

My opinion is that it's not so much a matter of muscles or strength, but most players have an ideal size or size range that works best mechanically on their face. If you don't experiment you may be missing it. For you, the larger size vibrates a better portion of your lip and going back smaller shows you how inefficient it was. Your lip just doesn't want to provide the necessary amplitude of vibration at that size... without significant time on it.

OK, so, granted that maybe I have noticed this - smaller mouthpiece than I'm used to will feel like it's getting in the way. At first, anyway. Now, what if I want to play a little bass trombone? On an old Reynolds 2B mouthpiece that's a perfectly middle-of-the-road bass trombone size - I guess I could get a larger mouthpiece if I really wanted to, but I'm sure never going to get, or want, anything as big as the Marcinkiewicz I've been playing lately. I gather people do this, switching back and forth between significantly different mouthpiece diameters, with the same face. I'd do it more myself, if I discovered any real point in playing bass trombone.

Some people also have a jutting chin and have to choose a rim size such that the lower rim rests somewhere in the region including the lower lip and the "dip" before the jut, to avoid having a "valley" that occurs before the lower part of the rim rests on the jutting chin, preventing a proper seal.

While I am far from a professional player, I have been playing for many years.

I find that I often switch mouthpieces when I switch horns. On my Meinl Weston 25, I almost always use a Conn Helleberg 7B, (I have a 120, but lost accuracy with it after giving it a serious extended try) On my Reynolds horns, I typically use a Bach 18. On my 6/4 Holton, I use a Stofer Gieb, and I really like it. (I do have to work much harder on low notes with it, but I have not decided that it is just my ability with the 6/4 horn.) If you read Lee's description of it, I think he is right on, just that I have had to work in building my low note security. http://tubameister.com/accessories/" target="_blank The Holton has a larger receiver so I have not done much experimenting with different pieces on that horn. On the Reynolds, the Bach 18 is just such an easy mouthpiece to play. They seem so natural that you just think of a note and out it comes, and thunderous bottom range. The Helleberg has been a great match for me on the Meinl, so this is what is in that case. I have a lot of different horns, and I fun with them, so that is one of the things I enjoy about playing.

Steve

As a trombone player, I find its easier to make a temporary move up than down in size. Moving from tbone to tuba when most of my time is on tbone is relatively easy. Moving from my usual tbone mpc to a smaller tbone mpc is difficult.

Some people can change mouthpieces all day long and some can't. I seem to only be able to change easily in one direction.

I don't see many tubists using Doug Elliott Modular mouthpieces, but a lot of bone players do. Maybe you should give one a shot. His system allows easy tweaks and fine adjustments if you use multiple mouthpieces, with his system, you can use the same rim and change underparts easily. Go see him at a show or carch him on a road trip sometime. Just contact him directly. Nice guy.