I've found most of the cheaper wigs are described as for an 'average' head size.
That is fine for my Teddybabe
, but not so fine for my Irontech 163 Ella head. My textile dolls head is also pretty small. The main bugbear I have with the too large wigs is the way they cover over the ears. If you are handy with a sewing needle there are tutorials on youtube for reducing the cap size, but I haven't tried it yet.
The seller on ebay I bought my first load of wigs from the caps were definitely smaller though than the ones I've had from Wiwigs, though both were an 'average' cap size, so there is variation. Unfortunately she no longer lists wigs.
I've more recently bought a Petite and 2 Mini Petite Natural Image wigs and one from Dimples (Petite). Natural Image do Large, Average, Petite and Mini (aka Ultra) Petite cap sizes. Dimples much the same. Natural Image have a head/wig sizing guide here:
https://www.naturalimagewigs.co.uk/Cap-Sizes/
At first I thought the Ultra Petite was the best fit of these, but now I think Petite is the best compromise for my Irontech 163 Ella head.
If you do a search for 'petite wig' on ebay you can be lucky and pick up the odd bargain petite sized wig, or Ultra Petite, if that is more suitable for you dolls head.