From the documentation:
coeff = pca(X) returns the principal component coefficients, also known as loadings, for the n-by-p data matrix X. Rows of X correspond to observations and columns correspond to variables. The coefficient matrix is p-by-p.
In your case, your data are 20*7 and the output is 7*7 as expected.
If you were expecting 20 coefficients, transpose the input.
[. . .] = pca(mArray.');
See the link above for a detailed explanation of the output and several examples.
