I know this sounds stupid, but can you try pumping it up a bit more? I thought my 65cm was too small at 5'4" but when I mentioned it to DH he just whipped up the pump and blew it up more! Its perfect now and more comfortable!
I got mine at Asda for a fiver and it came with a pump that worked! It's a 65cm exercise ball but does the job for a comfy seat and I'm 5ft5. My knees seem to be in the right place too, especially when I added more air. I'm only 15/16 weeks at the moment but my backs been niggling me and it's definitely helped