7. Why do some bulbs
heat up more than others?

Bulbs that heat a gas or a filament will heat up more than other light sources.

Explanation:  

Incandescent bulbs are consist of thin wire known as filament which converts electric current into light. These filaments are made up of tungsten metal. The filaments gets hot due to the passage of electric currents which turns the bulb up. Bulb starts glowing and produce light then.

There are many ways to convert electricity into light that is why some bulbs heat up more than others. Some of then are very efficient (converts electricity to light mainly) while others are less efficient (converts electricity to light as well as produce plenty of heat).  

For example: