Newer phones allow for a higher charging current, hence they get hotter while charging

No, that’s normal when charging, especially fast charging.

First define hot, how hot is the battery? Are we talking battery temps of around 40c or much higher? It’s especially normal for the area around the CPU to get hot because the phone isn’t trying to save power and it’s trying to do backgrounds tasks.

It can be any or all of the factors you just mentioned, also it could be trying to charge the battery faster than its own good for that model.

Also could be the type of phone case you’re using. Not all cases are made the same, but all of them keep the phone insulated to some extent or another.

Generally no, but yes. There is a multitude of factors to consider.

1. Capacity and shape of the battery. The former dictates how much energy, and therefore temperature, will enter the system in total. The latter for thermal dissipation.

2. The wattage which dictates how much of this energy flows in a given time into the system. If it is lower, then it will not get as hot. Get a lower rated charger to reduce temperature at the expense of required charging time.

3. The cooling concept and how effectively it dissipates energy. A better thermal design will allow for more wattage at the same temperature. Indeed companies will save here on resources often.

4. Software and regulation might have an impact on how the charging is controlled. I for example cap it at 80% for longevity reasons and therefore less energy will be in the device. Devices also have settings to activate faster charging.

5. Type of energy delivery. If you use wireless charging roughly half of the energy will become heat.

GSMArena says both phones take 10W power input.

One likely candidate is the type of power regulation used for charging. A Li-ion battery is about 3.0V when empty, and 4.2V or 4.35V when full. Basic USB power is 5.0V, or higher for various fast-charge technologies neither phone uses. To provide the correct voltage during battery charging, a charging circuit could use an expensive, bulky, efficient switched mode power supply, or a cheap, compact, inefficient linear regulator. The latter just turns the voltage difference into heat.

Are they charging at the same power input?

Charging systems can differ in efficiency, some may let off more heat than others. Same with battery cells, IR differs between models of cells and higher IR = more heat at the same charge current.

Direct charging like VOOC or PPS will also be more efficient than protocols like QC or PD as they minimize losses in the buck converter.

Only iPhone’s are getting so hot they are burning people. It’s poor engineering more than cheap parts, I imagine.

If I had to guess, the newer phone was designed to charge faster hence more heat.