Need help with sizing for a pillow

I’m making a pillow that has a cabled front and stockinet the back. The pattern says that it is to measure 16" across. The front is measuring 16" (cast on 109), the back is measuring about 15" across (cast on 72). imageimage
When I drape the front over the 16" pillow form it seems as if the cables are stretching out so it may even fit an 18" pillow form width-wise ( I’d have to knit more rows to get 18" tall then. So my questions are:

  1. Should I go with a 16" pillow form (let the front be loos) and redo the back with more cast on stitches than the pattern calls for? I’m getting 4.25 stitches per inch but somehow with the 72 stitches it’s only measuring 14.25"- which makes no sense to me.
  2. Should I use a bigger pillow form and let the cables stretch out a bit? Still would need to redo the back.
  3. Also there’s a single crochet border I still need to do on both the back and front. Will this maybe help prevent the cables from stretching out?

This is my first cable project so I really have no idea

The cables are going to have more stretch, Have you blocked the pieces to see what size you get?
What pattern are you following?

I haven’t blocked anything. I left the needles on the front in case I need to add more rows and go up a pillow size.
It’s a pattern from a Leisure arts pillow book I bought in a craft store

Either using a 16" or an 18" pillow will work. It’s really your preference. In either case you’ll need to re-do the back of the pillow.
The crochet border may help stabilize the cables but there’s more yarn in the cables and more give so they may loosen with time anyway.