How do I fix the pockets on women's jeans so they're bigger?

Yes, that is possible. If you have some sewing experience and a sewing machine it's not too difficult.  I can't post pictures so will try to describe the steps and hope it makes sense: 

Cut out the white cotton pocket bag from the yoke and front and leave the denim yoke -the part with the little coin/watch pocket. From the curved front cut the pocket only and leave the yellow topstitching and original seam intact. If you take that out this process goes from easy to very difficult.  Take out the seam that holds the two pocket pieces together. Trace along the cut lines on a piece of paper this is your Seam line. Extend the lower edge as long as you want and erase the original lower line. Now add another line half and inch outside the pieces. This is the Cutting line. Cut the new pocket pieces from a cheap lightweight cotton. (My local Walmart has lightweight cotton for making masks and it's super cheap) Sew the yoke pocket piece to the yoke. Sew the curved part to the pants next. Use a thread that matches  the denim. And a zigzag stiches will work best to hold the pockets d prevent the pocket fabric from fraying. Next sew the pocket pieces together to form a bag. First row on the seam line  should be a straight stitch a d the raw edges need to be enclosed in a zig zag stitch. 

If you can hand sew then use a strong backstitch to sew the pockets to the denim. To sew the pocket bag together use a close backstitch and use a slanted overcast stitch to roll and enclose the raw edges. If the sewing is visible on the outside of the jeans use a bit of embroidery thread to decorate along the curve.

If you don't know a needle from a pin and a back stitch from backseat and a pocket yoke from an egg yolk then get it done at a drycleaning who offers alterations and repairs or an alterations shop.  Enlarging jeans pockets is a common request. They need to make them large enough for modern phone.

Hope this helps. You might find a couple you tube videos showing this. If you don't then it's time for me to make one! 

take them to a tailor and have him fix it