I'd recommend ignoring all these suggestions and trying some out for yourself.

You should be able to return the ones that don't work for free within 30 days.

8bitdo is a good buy. especially if you ever want to use it with a switch.

Depending on what you're emulating, the 8BitDo ultracompact options might work for you:

8BitDo Micro

Basically turn your phone into a Gameboy, good for 90s games where you don't need sticks. The controller and clip together will cost you $35.

The new MCON seems to be the best solution currently available.

https://youtu.be/xFLtH4jLkZ8

https://ohsnap.com/collections/mcon

Do not get the Razor! I got one and have had nothing but problems and now it flat out just doesn't work. I bough the 8bitdo equivalent and it's awesome. https://amzn.eu/d/b1tmLi9, neither are that compact though. It looks like there are a fair few recommendations so I'm sure you'll find something, just dont get the Razor.

If you already have a BT controller (like any modern console's controllers) you could just get a thing that lets you attach the phone to that controller. That's what I did. They can be purchased for cheap, or printed yourself if you have a 3D printer.

I don't have any small form factor recommendations, but I can recommend - if you're comfortable going AliBaba/AliExpress - the Ípega brand. It's about £10, requires an app to configure, but extremely long-life battery and switchable modes. But the best controller for the rare few mobile games that support controller input natively is an Xbox One.. one, and any other made after that point

I do not have experience with any of those, but I do have a Backbone controller. It is a great inexpensive option and fit my iPhone 13 ProMax.

Gamesir hasn't done wrong by me yet. I can recommend the x5 lite over what you posted for size factor alone. I got a little case for it on the cheap and it can fit in sweatpants pockets (though not jeans). I think the bluetooth x2 comes with an even smaller case for free, though i prefer the usbc connection for pass through charging.

I did this same controller search earlier this year, and I can't really compare it to any similar controllers because it's the only one I've used, but I ended up going with the Abxylute S9 and I've been happy with it. I really wanted usb-c power passthrough and a 3.5mm headphone jack.

It's probably too big for what you're considering, but I think they sacrifice too much of what makes it usable and comfortable to go much smaller.

I too have been looking for something like this, for quite a while actually, and have been unable to find anything that is suitable. The 8Bitdo Micro looked great except it didn't have thumbsticks, and anything else was either way too big to be pocketable, or didn't have the proper amount of controls. Also, I would especially prefer something that keeps the device horizontal.

I did look at the GameSir Aileron X4, which seemed like a decent option but it seems too big to fit in my pocket. Has anyone tried this controller before?

I shall watch this thread closely.

I have a Razer Kishi and I love it, on mobile I use it with a pair of Hammerhead wireless buds to play xcloud. It is one of the most compact options and the USB connector supports power through to the phone. The small size also means the sticks, buttons and triggers are also small. If you use a case on your phone it will have to come off when you slot it into the pad, there's no room unless the rubber holders are removed, when I did this the setup became wobbly. One last thing will be to check that your phone will fit, a Galaxy S23 Ultra is too big.