I have a lot of Crocs in a variety of women's styles, from the traditional, original Croc, to Mary Janes and sandals.  In all others, I have worn an identical size.  With the Mammoth Crocs, I found I needed to order one size up for comfort, because the lining is inserted into the normal size Croc, taking up space, and my toes felt squished.  Once I did that, they were perfect!  They are really warm and comfortable, and let me wear Crocs all year round - a far better solution than just wearing warm socks with regular Crocs.