Lorica, the material Sidi uses for their boots, does not stretch or shrink; even it if gets wet. Borrowing from the web,
Why Lorica? Because it has all of the upsides of leather with none of the negatives. Lorica is supple, conforming, highly breathable and will not stretch. In addition, Lorica is water repellent, highly abrasion resistant, easy to clean and will not rot or dry out.
I have different models of Sidis and I've had no issues. Most comfortable boot I've ever worn.