A recipe might call for one large onion or two small eggplants. When cut as directed it seems to yield way more than the recipe intended. I got the two smallest eggplants in the store and after cutting one I had more than enough for the recipe. It almost seems like the people who write the recipes only have access to veggies that are smaller than average. Is there a reason why their large onion is equivalent to my small to medium onion?
The size of the veggies can be due to location only to an extent. If their seasons are longer for cultivation to harvest then the plants may experience better growth.
Other factors can also contribute. The use of fertilizer for example, or if using hormones and pesticides, etc... all effect the outcome in size, quality, taste and possibly even firmness or even color.
As in the answer and comments already posted, location, growing conditions, etc. all play a part. But one of the most important things to keep in mind is that there are many varieties of each type of produce.
Most often, in recipes, you are not provided with varietal information. So you have no way of knowing which variety may have been used and this can result in a different yield than the recipe stated.
For example a medium Vidalia onion will not be the same size as a medium Spanish onion. Or as in asparagus, some varieties are small, some large.
Also, when purchasing produce, we do not always know what varieties we are getting as the information is not available at the point of purchase.