I would say that in general Barbie and Candi heads are similarly sized and the dolls are similarly proportioned. They are both 11.5" dolls so their head sizes are comparable.

Here is a link to photo reference (not mine) showing a Mikelman retro Candi (1997?) next to a vintage Barbie TNT (late 1960s?): https://www.flickr.com/photos/romitagirl67/11765601263/in/photostream/ They look similarly sized to me.

I hope this helps.