You can read  font-ini.mkvi :

%D \macros
%D   {setupbodyfont,switchtobodyfont}
%D
%D The next two macros are user ones. With \type{\setupbodyfont}
%D one can set the document bodyfont size, font family, style
%D and/or options defined in files, for example:
%D
%D \starttyping
%D \setupbodyfont[modern,12pt,roman]
%D \stoptyping
%D
%D This command affects the document as a whole: text, headers
%D and footers. The second macro however affects only the text:
%D
%D \starttyping
%D \switchtobodyfont[10pt]
%D \stoptyping
%D
%D So we've got:
%D
%D \showsetup{setupbodyfont}
%D \showsetup{switchtobodyfont}
%D
%D Both macros look alike. The second one also has to take
%D all kind of keywords into account.

If you follow switchtobodyfont you can see

unexpanded\def\font_basics_switchtobodyfont#specification%
  {\edef\m_font_step{\bodyfontvariable{#specification}}%
   \ifx\m_font_step\empty
     \font_helpers_set_font\zerocount{#specification}%
   \else
     \font_helpers_switch_bodyfont_step % so we have a fast [small] switch
   \fi
   \the\everybodyfont
   \the\everyswitchtobodyfont}

\unexpanded\def\switchtobodyfont[#specification]% could become an ifx
  {\doifsomething{#specification}{\font_basics_switchtobodyfont{#specification}}}

The token register \everybodyfont  and

\everyswitchtobodyfont take care for spaces:

(still in font-ini.mkvi)

%D   {everybodyfont,Everybodyfont,everyglobalbodyfont}
%D
%D Every change in bodyfont size has conseqences for the baseline
%D distance and skips between paragraphs. These are initialized
%D in other modules. Here we only provide the hooks that
%D garantees their handling.

%D At the system level one can initialize thing like:
%D
%D \starttyping
%D \appendtoks \setupspacing \to \everybodyfont
%D \stoptyping

In core-def.mkiv

\appendtoks
    \synchronizelocallinespecs
\to \everyswitchtobodyfont

while  spac-ver.mkiv

\unexpanded\def\synchronizelocallinespecs
  {\bodyfontlineheight \normallineheight
   \bodyfontstrutheight\strutheight
   \bodyfontstrutdepth \strutdepth}

--
luigi