Consider the following example:

\documentclass{beamer}

\usepackage[backend=biber]{biblatex}
\addbibresource{biblatex-examples.bib}

\usetheme{moloch}

\begin{document}
  \begin{frame}
    Normal Text \fullcite{knuth:ct}\footfullcite{knuth:ct}\footnote{Normal Footnote Size}

    \Tiny Normal Text \fullcite{knuth:ct}
  \end{frame}
\end{document}

In default beamer themes, the font size of the citation can adapt to the surrounding context, e.g. to the size of footnotes or text around it.

One could avoid the problem by not hard coding the size of the bibliography items:

At least for the ones with \normalsize, the hard coded sizes don't seem to have a lot of advantages, as they are normally uses in normal size anyway, but cause problems like the ones mentioned above.