\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes,positioning}
\begin{document}
\begin{tikzpicture}
\node (A) [cylinder, shape border rotate=90, draw,minimum
height=3cm,minimum width=2cm] {};
\node (B) [cylinder, shape border rotate=90, draw,minimum
height=3cm,minimum width=2cm, below=0.3cm of A]
{Wider?};
\node (C) [cylinder, shape border rotate=90, draw,minimum
height=3cm,minimum width=2cm, below=0.3cm of B]
{Why is the Ellipse wider?};
\end{tikzpicture}
\end{document}
Generates:
The ellipse at the top of the cylinder node gets less oblate as the text inside the node gets wider. How do you control this?
I would like to keep approximately the same aspect of the ellipse despite stretching to fit text.

aspect? – cfr Mar 29 '17 at 23:04why is the ellipse wider?, what do you mean? It is wider, clearly, because the text you are putting in it needs more space, so the cylinder has to expand to accommodate it. That is just how nodes work: unless you override it, they are sized to fit their contents. If the cylinder is wider, obviously the ellipse has to be wider, too, else it would no longer be a cylinder but two lines and two detached ellipses. – cfr Mar 30 '17 at 14:39