The truth is pores can: dehydrate, soften, enlarge, become impacted, break down, become hardened, stretch out, and become flaccid.
If they're kept clean and exfoliated, I've seen them go back to 'normal'... But honestly, you cannot make them smaller than what you were born with. Everyone has pre-programmed genetic pore size. Certain conditions can affect pore size and pore health.
Pores can and will become larger depending on the circumstances. Generally, we have larger pores in areas of the face that have more prolific oil production - usually the "T-Zone" (forehead, nose, chin, and sometimes the cheek centers). Some people are born with large pores. Sebum (oil) is the skin's natural emollient which helps retain the skin's hydration (water). Without proper hydration, the skin does not function well. The pores become keratinized - resulting in thickened and hardened skin. Lack of hydration/water greatly impacts on how the skin functions.
Stop trying to scrub and scrape them into shape. Do not squeeze, or pick at them with tweezers. Stop stripping away their natural protective pH and lipid matrix.
Instead, feed them, water them, protect them with a moisturizer (and NOT an oil-free one - blog article forthcoming). It is a good idea to have the impacted plugs and blackheads removed from the pores professionally without hurting them. When things get out of control, get a professional deep cleansing facial to help you get your skin back to a healthier stasis. Have your esthetician educate you on how you can maintain cleaner pores, deal with congestion and excess oil at home; without causing over-dryness, stimulate unnecessary oil, or cause damage and inflammation.
Oh, and blackheads are not dirt in your pores. The dark color is oxidized sebum. (Once in a while, I will see pigment from mineral makeup fill up pores.)
Love your pores and they will love you back.