Some months ago, I found a recipe for cookies and made them in a big oven; they were delicious and cooked very well (soft cookies) at 180 °C (356 °F) for 7 minutes.  I recently tried the recipe again, but I have no longer a big oven, so I used my little one (with heat source at the top and bottom). The cookies were still almost liquid after 15 minutes, and I though that was my oven that was too old and not keeping a good temperature; I tested on another oven that is new, and I got the same result.  Does the little oven heat less than the big one? If yes, what do I need to do to adjust the temperature and/or the time? If not, what am I doing wrong?  The recipe is:        125 grams of soft butter   125 grams of brown sugar   175 grams of white flour   one egg   1 cc of baking powder    100 grams of chocolate chip         Mix all except chocolate chip. When you have a homogeneous substance, add the chocolate chip.   Bake them 7 minutes at 180 °C. they're still very soft after that delay but they will harden.  
My guess:  With a little oven you probably don't preheat as long, and when you open the door you let ALL the hot air out. You need to reheat the air in the oven after the cookies go in for them to cook. Because the hot walls and rack are much smaller (and probably not as well heated to start with), it's harder for them to reheat the inside of the oven.   So the temperature is lower when you start the cookies. Some (maybe a lot) of the cooking time is spent getting the oven, the baking sheet and the air back up to temperature. Cooking them longer or at a slightly hotter temperature should help with this. 