That's true. But on the other side you have the motor sticking out, so it doesn't make as much of a difference to the overall size on that side.
If you look at what I drew up it's similar to ThomConcept but with the addition of the chip guard. I am more of a fan of the high rail design as opposed to tall gantry risers. ThomComcept's design looks like it has tall gantry risers, but it doesn't, it's a high rail design, he just spaced his gantry bearings very far apart.
I really think that it's a good idea to look at what has been proven to work, in particular because you said that you want to cut mostly aluminum. Can you find some examples of designs that cut aluminum well and have really tall gantry risers like yours where the ballscrew is under the machine? It's not a rhetorical question...I don't know, have you found any?
I personally don't like anything driven from under the sides of the machine because it prevents the machine from being bolted down along the sides, which increases the rigidity on the long axis.
Also there is nothing preventing you from doing a dual drive under the machine if that is what you want to do. It's not what I would do, but it's not my choice. Ultimately, all anyone here can do is offer an opinion. Sometimes those opinions are good, and sometimes they are not.
Since it is so important to you to keep the footprint as small as possible....I suggest starting to draw in things like your drag chains, etc to see how this effects the size of your machine.
Designing a machine from scratch takes so much work and rework in the CAD files to get it right. That's why I've been saying to hold off on buying any parts until everything is finalized the way you want it. It's not a time to rush things.
You should check out the Avid CNC benchtop PRO design. It looks like they have made it to take up as small of a footprint as possible for the amount of travel. Ballscrews driven on the sides, as opposed to from underneath, and that would be my choice. I will have more to say on this later.
https://www.youtube.com/watch?v=ZUNpi0lTg5c
I have a bunch more things to say, but that will come over a few more posts. Hopefully my comments are helpful, and not too annoying. I'm certainly not trying to come off as a know it all, because I certainly don't know it all.
That describes mine as well. So true. I added the bold to CAD.
This is an interesting build as well, but has a moving table design.
https://www.youtube.com/watch?v=WvOhgM5uBF4