Ok, well as a generalisation, the thinner the wire, the higher the resistance and the thicker the wire the lower the resistance.
Think of resistance as the wire resisting the electricity running through it, trying to stop the electricity flowing. This will make the wire heat up.
Thin wire gives less space for the electrons to flow through, whereas thick wire allows the electrons lots of space.
The more resistance (thinner wire), the easier it is to heat up the wire (low wattage), but a smaller amount of wire is touching the cotton (less vapour), so it's more suitable for MTL.
The lower the resistance (thicker wire), the harder it is to make it heat up (higher wattage), but a larger amount of wire is touching the cotton (more vapour), so it's more suitable for DTL.
So MTL coils tend to be small, whereas DTL coils tend to be more chunky.
