The following is more of an extended follow-up comment to Przemysław Scherwentl's answer than a new answer. Too long to fit into a "comment" box...
Borrowing (stealing?!!) some code from @PaulGaborit's answer to the question Why do all symbols in $x \in X$ have their own baseline?, here's a visualization of the size and positioning of the symbols
+, using the default Computer Modern math font:
\in is quite a bit larger than either
\varepsilon and that the two math symbols are aligned on the math-axis line. Because the math-axis line is slightly above the half-way mark between the baseline and the x-height line, the upper edges of the
+ symbols protrude further above the x-height line than their lower edges protrude below the baseline.
Addendum: Just for comparison, here's the output of the same code, but with the math font
newtxmath (which is based on/derived from Times Roman) in use instead of Computer Modern:
Interestingly, the symbol
\in is a bit smaller in the newxtxmath font than it is in Computer Modern; even so, the
\in symbol is still clearly larger than the glyph for
\epsilon. And, because
\in is centered on the math-axis line and its lower edge touches the baseline, its upper edge clearly protrudes over the x-height line, whereas that of
\epsilon does not (by design, of course). Another difference between
\in is in the position of the middle bars: It's on the math-axis in the case of
\in, but well below it in the case of
Either way, it's clear that the font designers were well aware of the need to keep
\in easily distinguishable.