Heap sizing (and possibly resizing) in Hotspot GCs needs to take various alignment constraints into amount.
One of them is that Hotspot supports only heap sizes that are aligned to the area a full vm page of card table entries spans. Hotspot does not support card tables which entries do not correspond to non-committed heap memory (1).
All collectors but G1 take that into account in their sizing decisions (2).
(1),(2) needs to be verified before starting work.
This bug has been observed when fixing bug 8014408, see also http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2013-May/007181.html