I've run into this as well and believe it is a bug. While you can't avoid it, you can repair it.
In the Layers panel, click the chainlink icon between the mask and maskee. This breaks the size relationship between the two objects.
Select the mask in the Layers panel, then either use the Scale tool or the W and H fields in the Properties panel to correct the size.
Relink the two objects by clicking in the chainlink area.
Thanks mate worked great!
We've stayed away from Fireworks CS1 - CS4 because it just seemed to have so many bugs on the Mac. Do you know if CS5 is much improved in this area?
CS5 is much better than CS4. I'm pretty happy with it in general. in the 5.1 update some other issues were also addressed. You can check out what was tweaked on the FW Team blog: http://blogs.adobe.com/fireworks/