What Causes Enlarged Pores?
Two main things:
Genetics and age.
If you come from a family with thick and oily skin then it is odds on that you will have the same skin. This type of skin has a tendency towards enlarged pores.
The sun plays its part too - prolonged exposure over the years causing sun damage together with decreased elasticity can make pores more obvious.
If you have had long term blackheads this causes the diameter of the pore to expand. And even if you religiously remove them and keep the area clean they are there to stay.
Heat and humidity can be a factor certainly if experienced for a long time. This is why pores are often very visible on Asian skins.
If pores are dirty they will also appear larger. So keep it clean!