It is about the recipe and the firing temperature. This is Plainsman BGP, a terra cotta, with 30% dolomite. Note the "DSHR" column in the SHAB test data (third last column): The drying shrinkage still averages over 7% even with the 30% dolomite, so BGP is very plastic. Notice the "FSHR" column, it is negative for the first five test bars fired at cone 05-01, that means the bars grew in size! But notice the shrinkage hits 0% at cone 1 (bar #6), by cone 2 the trend has reversed to 0.3% shrinkage. Amazingly that #6 bar is approaching vitrification, it is strong. This body was intended as a high-porosity ceramic at the lower ranges, at cone 04 is so porous that it can be used as a filter.
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.