Every time I eat at Cascal, I don't understand why they don't offer smaller portions at the same prices. They could.
Most small plates restaurants recommend ordering 2-3 tapas per person plus 1-2 full-size dishes for the table. At Cascal, if each person ordered just one small plate and no full-size dishes, you'd be stuffed. Even what Cascal describes as "small" is big. For example, the "small" paella alone could easily feed three people.
Giant portions aside, the food itself is good. The tapas sampler is a great deal ($20), easily feeds four people, and allows you to taste half of the tapas menu. I also like the wild mushroom empanadas with manchego cheese and truffle oil, and you can't go wrong with any of the ceviches. Desserts are my only complaint. The portions are large, but the flavors are nothing special. Go to one of the many cafes along Castro Street instead.