pdflatex + includegraphics pdf file size very large

I have two PDFs whose sizes are

pdf1.pdf 1000kb
pdf2.pdf 700kb

Each have same number of pages and I have to make a new PDF with pdf2.pdf and pdf1.pdf mixed in a single page.

I am trying to put each of their pages together in a single page as follows:

documentclass[version=3.21]{scrartcl}
usepackage{geometry}
usepackage[automark,headsepline=false,footsepline=false]{scrlayer-scrpage}
setlength{parindent}{0pt}

begin{document}
% PAGE 1
KOMAoptions{paper=432pt:506pt}
recalctypearea
thispagestyle{scrheadings}
newgeometry{layoutwidth = 432pt,layoutheight = 506pt,left=4mm,right=4mm,top=0mm, bottom=0mm,footskip=1mm}
hspace*{-4mm}%
includegraphics[width=432pt,keepaspectratio,page=1]{pdf2.pdf}%
par%
vspace*{5mm}%
includegraphics[width=textwidth,keepaspectratio,page=1]{pdf1.pdf}%

%PAGE 2
%....
end{document}

The size of the pdf is 38.4 Mb. Why is it so big?