I have been buying from QVC for many years and I don't use the size chart. Instead I go to qvc.com and check the measurements for each individual garment by clicking on "click here" in the description before I buy anything to be sure it will fit (specifically for me the the low hip circumference and the inseam).
If I had to compare the 3 lines, I find Isaac's pants run smaller, Susan's run large and Denim & Co, just right. I bought Denim & Co for years before I tried Susan or Isaacs. The Denim & Co Capris(A14924), Crops (A14925)and Full Length (A51961) are semi-fit and good casual pants, I always were a large and they are comfortable and not too tight or loose. I tried the Susan Graver Lustra and Liquid knit pants and I can't wear them. I'm petite and I just drown in these, the rise is way off for me and comes down to my knees and I would have to hike the waistband up just under my bra. I recently tried her Milano knit and they are the first pair of her line that I can actually wear as the rise is do-able but still not really impressed. As for Isaac's pants - I LOVE them. Some of his pants I can order a large like the knit denim which I wear like a legging but in his 24x7 stretch I size up to an 18.
I hope this helps.