I think that if you sewed three 2.5in strips together with quarter inch seams, your little block shoud have measured 6.5in. You must be taking larger seams. When you look at that little block, from the front, finished, the first strip should measure 2.25in, the middle strip 2in, and the other edge strip 2.25in. , total 6.5in.
If you have four 6.5in blocks sewn together, then the completed block should then measure 12.5in square.