Android

On a nice technical level, heat and space. On a sales level, upselling. Larger phones dissipate more heat. Heat is bad for computers/phones. A 45w quick charge (which is dumb to use, fyi. Fast charging degrades batteries faster) has a larger amount of circuitry in the phone that takes up space to regulate and monitor the charge.

Beyond that, there's a nice little formula used for how fast you can safely charge a lithium battery, and that formula directly ties into battery size. Essentially, for every 1000 mah worth of battery, the battery pack can handle about a 1 amp per hour charge (roughly). So the bigger the battery, the higher the wattage and amperage you can somewhat safely go. This is the main reason the s24 won't have 45w fast charging. It has a 4,000mah battery. It's too small a battery to even use the 45w fast charge without degrading the battery too fast. The s24plus has a 4,900 mah battery. That's big enough to handle a 45w charge (though again, if you want your battery to last as long as possible, you want to disable it in settings.).

Now aside from just up selling, smaller phone= smaller parts needed. Your thermometer needs the hardware inside to support it and that has to go on the main board. Your smaller phone has less open space to fit it on there.

Then things like video boost while may be just an upsell, could also be because recording very high quality video is an intensive task on your phone so it causes more heat and drains your already smaller battery faster, which generates heat as well, and your smaller phones can't get rid of that heat as quickly.

Next, I had seen you mention 1440 screens. That means across the whole screen there would be something like 2,960 pixels/dots of light in one direction and 1440 in the other. The more dots, the more resources (processor use etc) you need to power them, so your battery drains faster and phone heats up more. But the smaller the entire screen is, the less dots you need to make it look the same to you from the one or two feet away from your face you hold your phone. Smaller screens don't really need a 2960 x 1440 screen. You can't tell much of a difference on a screen that size and it would just make the batt drain faster for almost no reason. I have a large screened Note 20 Ultra that has a 1440 screen and I don't even use it. I turned it down to 1080 because even on my larger phone the viewing quality wasn't enough of an improvement to trade off from how long my battery would last on a charge.

Ram is mostly a money grab, but each ram chip does also use more battery as well.

In summary, for some things it's a money grab, for others, there's good reason it's that way. But it's also become a niche market for people who want the high end stuff in a smaller phone. You're a minority. Cramming all the things in a smaller space is harder and therefor more expensive. There just aren't enough people who are willing to pay a premium price to get the highest end phones that want a small phone, so no one makes them. There isn't enough money in it. I believe you said in here you wanted to buy a phone for around $750, while top end phones are double that price, so not even you are willing to pay $1500 to get a 6" phone with all the best stuff packed inside.

Hope this helps. Cheers