The behavior you describe only occurs when the TrueType Palatino is
installed. The reason this happens is fairly straightforward. One of
the main features of TrueType is to provide an "as readable as possible" screen
representation of a font, in the size selected.
In some cases, TrueType will decide that it is not possible to bold a
font and still have it be readable. This is the case with Palatino.
However, this particular trait does not always appear to be logical.
For example, Palatino 4 can be bold on the screen and it is certainly
not readable.
In any case, selecting to bold Palatino will change the spacing of
the font to preserve the margins for the document.
There is no way to change this╩behavior.