Minecraft seem to use the glyphs from code page 437 (aka CP437, OEM 437, PC-8, MS-DOS Latin US) for its default font. Wikipedia says that when using this code page the German sharp s (ß) should share the code point (and therefore also the glyph) with the Greek beta (Hex: DF / Decimal: 225).

However, Minecraft does't do that as you can see on the screenshot incorrect-german-sharp-s-in-chat.png. As it uses the unicode font renderer, the German sharp s (ß) is displayed way too small. By comparing the screenshot to what your browser will display in a monospaced font, you'll see the difference more clearly:

schließlich

The file default.png you can find in minecraft.jar/font contains a Greek beta glyph. The beta glyph looks a bit too bold but I'm sure that it would still look better when using that one for the German sharp s than it currently does. (See minecraft-default-font-characters-with-highlighted-german-sharp-s.png.)

—

Some additional background information about the German sharp s: The ß glyph can be considered somewhat of a normal letter in the German language. It is used very often. Therefor by displaying the German sharp s way smaller than other letters, a visual break in chat appears quite often - which is an unnessesary annoyance that could be fixed easily. (By the way: The other normal German non-ASCII glyphs öäüÖÄÜ work as expected in Minecraft as the code page 437 is applied properly there - it's just the ß.)