A petite cap size is defined as a circumference measurement of < 21". This is a range, however, since most wigs can be adjusted in or out around 0.5" - 1" (depending on the brand) at the nape via the adjustable straps in the back.
An average cap size is also a range, with the median being a 21.5" circumference measurement. The typical average range is around 21" - 23".
As you can see, there is some overlap. This means that if you are on the cusp of an average cap size (what manufacturers like to call "petite-average", or about 21"), then you should be fine in an average cap from most brands. The cap may be a little loose, but with adjustment, the wig should fit adequately enough to be wearable.
However, if you are on the smaller side of this petite range (anything less than 21"), you should avoid average capped wigs. They will most likely be too bulky and will not fit as securely as you need for daily use.
These wigs can only be altered in a way to sufficiently make them fit you by an expert seamstress. The process involves cutting the interior of the wig cap at the seams and reconstructing it so that the entire wig becomes smaller in all areas: front to back, ear to ear, and at the perimeter. The wig will generally also need to be restyled and / or thinned by a stylist after this process as well.
CysterWigs does not offer these services at this time. We highly recommend utilizing the services of a competent brick-and-mortar wig boutique in your area if you are considering altering an average capped wig for your petite head. It is often quite a large undertaking, and can cost quite a bit of extra money . . . just so you're forewarned!