LaTeX Font Warning: Font shape `T1/cmss/m/n’ in size <4> not available
LaTeX Font Warning: Size substitutions with differences
Apparently, the compiled pdf was O.K., but the waning always made me uneasy. I just could not be reconciled. It turned out that Beamer defines \Tiny and \TINY font sizes that some fonts do not provide. Using a font package such as lmodern that provides these sizes solves the problem.