What happens to wine

So what happens to wine if you dont top it off with and leave some good 4 or 5 inches from the top of your carboy? Will this cause the wine to lose some of its alcohol? I bottled my wine last night and I had a glass to taste and I thought it was really weak...it has a watery taste to it. This is weird to me because a couple of weeks ago while wine sitting in my carboy all topped off I took a sample and i thought it was good and strong wine..but then I decited to rack the wine one more time before putting to bottle so I did and this time I did not move all the wine since I wanted to leave as much yeast behind as possible and by doing that I had a good 4 to 5 inches of free space from the top of my carboy. I thought i was going to bottle the wine the very next day so I didnt pay much attention but it I got really busy at work and ended it up leaving it for a week. Last night I finally bottled it but it was really weak...did i ruin the wine???

Maybe the hot alcohol taste of the young wine has started to age out and that's the taste difference but hopefully one of the wine pros here will enlighten us...

It won't get weaker (or stronger). The air in the headspace can cause oxidation. The wine can change color and get a "sherry" flavor to it- that's the trademark of oxidized wine.

I usually top off with a similar wine, white wine for light wines, red for dark wines, or with water if that's built into the recipe formulation. I often go from a 6 gallon carboy to a 5 gallon while racking, and keep the remainder in a small wine bottle just for topping up.