

Pin pitch is pin size and/or spacing. With physical plugs, you start to hit limitations with how small the wires can get while still being durable enough to withstand plugging/unplugging hundreds of times.
Drop losses. (I am keeping this at an ELI5 [more like ELI15, TBH] level and ignore some important stuff) Every electronic component generates heat from the power it uses. More power used usually means more heat. Heat requires physical space and lots of material to dissipate correctly. Depending on the materials used to “sink” (move; direct; channel) heat, you may need a significant amount of material to dissipate the heat correctly. So, you can use more efficient materials to reduce the amount of power that is converted to heat or improve how heat is transferred away from the component. (If you are starting to sense that there is a heat/power feedback loop here, it’s because there can be.) Since a bit of power is converted to heat, you can increase the power to your device to compensate but this, in turn, generates more heat that must be dissipated.
In short, if your device runs on 9v and draws a ton of power, you need to calculate how much of that power is going to be wasted as heat. You can Google Ohms Law if you would like, but you can usually measure a “voltage drop” across any component. A resistor, which resists electrical current, will “drop” voltage in a circuit because some of the current (measured in amperage) is converted to heat.
I kinda smashed a few things together related to efficiency and thermodynamics in a couple of paragraphs, but I think I coved the basics. (I cropped a ton of stuff about ohms law and why that is important, as well as how/where heat is important enough to worry about. Long story short: heat bad)
(For others reading this, this is a perfect followup to my comment here explaining the “why”, while this is an excellent view into the “how” and picks up the bits I dropped about Ohms Law.)