You will want them to all be the same size or the whole quilt might look a little wonky. The difference is probably that your seams may not be exactly 1/4 inch or a tad less, or the seams haven't been pressed open correctly, or the pieces were cut inaccurately, or whatever. This was the hardest thing for me when I started quilting... mastering the 1/4 inch seam - or better a scant 1/4 inch seam. I still often have blocks that are not exactly the same size. I try to keep it within an 1/8 of an inch or less. Anyway, it looks like you will probably be able to square the blocks up. If there is only a 1/4 inch or less difference, you can ease the difference by sewing the seam with the side that is a bit longer on the bottom. That way the feed dogs will help pick up the extra. It looks from the photo of the quilt like you could adjust the cornerstone size a bit if necessary. Good luck with this pretty quilt!!