Xelatex: Section fontsize

Completing your fragments to an example document:

documentclass[12pt]{book}
usepackage{titlesec}
%usepackage{lmodern}
titleformat{chapter}{centeringbfseriesfontsize{20pt}{0}selectfont}{Chapter thechapter.}{10pt}{}

titleformat{section}{bfseriesfontsize{16pt}{0}selectfont}{thesection}{10pt}{}
titleformat{subsection}{bfseriesfontsize{14pt}{0}selectfont}{thesubsection}{10pt}{}

showoutput
begin{document}

section{aaa}
zz
end{document}

If you use this with pdflatex then you get the warning

LaTeX Font Warning: Font shape `OT1/cmr/bx/n' in size <16> not available
(Font)              size <17.28> substituted on input line 12.

and the showoutput log confirms the aaa heading uses 17.28pt font

....OT1/cmr/bx/n/17.28 a

This is due to historic compatibility issues forcing Computer Modern to be used in a fixed set of sizes. You can use the fix-cm package or choose a different font package eg Latin modern (commented out in the example above).

However you indicated you are using xelatex, which already defaults to the OpenType Latin Modern and 16pt font is used in that case:

....TU/lmr/bx/n/16 aaa