I'm always game to buy a new gun, and I'm as good as anyone at coming up with reasons I "need" it. I have to ask what you are hoping to gain by getting the new smaller gun.
If it is size and ease of concealment then I would look at either a bobtail or a CCO size gun. If all you do is shorten the slide, then in my experience, you gain very little in terms of concealability, if carrying IWB. The bobtail grip tends to hide a little better and the CCO size gun gives you a shorter grip that hides away a little easier.
Weight: If you are looking for less weight then a commander size gun is lighter and a CCO lighter still. You may want to look at an aluminium frame gun if weight is the big concern. There are trade offs, and some people prefer an all steel gun. The weight savings of an aluminium CCO over a 5" steel govt. model is notable. The former weighs around 1.6 lbs, the latter 2.3 lbs. Above someone stated a steel CCO is about 32 oz the aluminium frame d one is about 25.6.
However, a note about weight. If one hadn't already I would always recommend trying a real good gun belt (like an Ares ranger) and holster first before the effort and expense of a new gun.
An officer sized gun will give you gains in both size and weight. Conventional wisdom, and the opinion of a lot of very knowledgeable folks is that officer size guns as a whole are less reliable. I would be more inclined to get something like a springfield EMP, that was somewhat of a ground up redesign. That is only an option if you are not set on a .45.
The gun I like the most, for CCW, that fits your criteria is the Dan Wesson CCO. It comes with a lot of features I like, and Dan Wessons are good guns.
http://www.cz-usa.com/products/view/dan-wesson-cco-bobtail/