MikeSid is a bit off......
Garlic has a defense mechanism. When it is damaged (e.g., crushed, chopped, squished, etc.), the enzyme alliinase converts the alliin in the cloves into allicin. More sulfur compounds are formed when alliinase comes in contact with alliin than just allicin. These compounds are also part of the garlic flavor we know. The only way to make crushed garlic acrid is to brown it too much or burn it. Cooking it destroys the allicin but the other sulfur compounds remain giving us the wanted garlic flavor.
All that happens when you crush garlic in a press is that you increase the amount of alliin converted into allicin (along with the other associated sulfur compounds) compared to what you would get with crushing it with a knife. That's it. Nothing more than that.
So, if you use a press, you need less garlic. Regardless, it is often recommended to let the crushed or squished garlic sit for 10 minutes before using.
Here is so more useful information:
The Science of Cooking With Garlic - Real Food - MOTHER EARTH NEWS