Why is the line spacing different between these paragraphs?

In the following two paragraphs, the line spacing seems to be wider in the bottom paragraph, and I can't for the life of my fathom why:

The code for this part plus a little before and after is

{\small
\noindent\vb{Systems of First-Order Linear ODEs}\\
%parts omitted for brevity

\vspace{10pt}
\noindent\vb{Sturm-Liouville Theory}\\
Introduction and motivation to Sturm-Liouville problems, SL eigenvalues problems,
expanding functions in eigenfunctions, convergence of expansions, applications to non-
homogeneous BVPs and linear PDEs, transforming ODEs to SL form, singular SL problems and
the Bessel equation, solving the wave equation in 2D.

\vspace{10pt}
\noindent\vb{The Laplace Transform}\\
Introduction to the Laplace transform, calculating the Laplace transform for various
simple functions and products, using the Laplace transform to solve second-order linear
ODEs, and ODEs with discontinuous or impulsive forcing, convolutions, application of
convolution theorem to ODEs and integral equations.
}

\pagebreak
\tableofcontents


• why the low level markup \noindent\vb{Systems of First-Order Linear ODEs}\\  instead of a section heading??? You also get warnings about the mis-use of \\ which should never be used at the end of a paragraph. Dec 4 '21 at 9:56

Always include a blank line before ending the scope of a size change. The scope of \smallchanges at the }but the second paragraph has not been set yet so it is set with the baselineskip restored at the }designed for \normalsize, even though it used the small font.

Unrelated but

\vspace{10pt}
\noindent\vb{The Laplace Transform}\\


generates warnings due to the mis-use of \\ and does not include any of the features LaTex normaly uses for a heading such as preventing a page break after the heading. It would be better marked up as

\section*{The Laplace Transform}

• Adding the blank line at the end of the \small section helps, thank you. Dec 4 '21 at 10:26