Fresh air?

Not odd at all imo. Larger pot, more light and outdoor weather which toughens up and cleans foliage. Hardy plants invariably do far better outside in the conditions where they evolved.

I have large and small pots outdoors, and the small pots outside still have larger leaves than the pots inside. So pot size isn't the answer.

Hmm, outdoor weather toughens them up? I actually get more growth indoors, but the plants outdoors have larger leaves. Not clear who gets the badge for more toughness. The one with a bigger plant, or the one with bigger leaves. After a freeze and 100F temps, the outdoor plant looks pretty sketchy. I guess evolution didn't help that any.

I promise you, the outdoor plant gets better sunlight, air quality, humidity, etc. The inside environment is a handicap for plants, especially outdoor loving herbs.

Dan, you may get more growth but that doesn't equate to toughness. That is why one has to harden off plants being moved from inside to outside. Indoors plants may appear to be growing more but they are very probably reaching for light.

However sketchy the outdoor plant looks it will very likely recover because it is used to the conditions. Catnip is a hardy perennial. It is adapted for outdoor living.

The question wasn't about toughness. The question was about leaf size. The outdoor plants do NOT get more sunlight (probably less, in fact). Air quality? Can't say my indoor air quality is much worse than outside. The outside is where that air comes from! But yes, it is considerably more humid outside. Interesting that higher humidity might drive larger leaves.

Unless you have indoor plant lights or a very gloomy garden I can't think of any situation where an indoor plant would get more light than an outdoor one. Those of us who put our house plants outside in summer know that they always thrive there. Tough plants are healthy plants. Healthy plants have larger leaves. Outdoor plants get wind and rain, indoor plants do not. But I'm not sure why I'm replying since you clearly know better.

My pot outdoors is under about 80% shade by trees, and it has another shrub hanging over it. Yep, my indoor window gets more sunlight. Lots more.

In my climate, you put houseplants outside in the summer if you want to stress them.

Sorry, but about these things in particular, I do know better.

But I will consider the puzzle of leaf size unsolved.