I'm going to make this more complicated than it has to be, but it's helpful info.
OK so regarding 1911s and magazines we have to point out that originally the full size 1911 held 7 rounds of .45ACP and the Compact or Officers 1911 held 6 rounds of .45ACP. A while back, some genius figured that the skirt of the follower didn't really need to be as long as it was and that you could chop it down to add one more round to the magazine and have 8rds in a full size and 7 rounds in a compact.
For those that don't know what a follower or a skirt is, here's a photo.
Left: Wilson 47D 8round, Center: Tripp Cobra Mag, Right: Older Springfield Armory 7rd magazine.
The follower is that thingumabob that you have to push down in a magazine to put the cartridge in it. The skirt is the part of the follower that goes around the magazine spring and will make contact with the bottom of the mag to prevent over compressing the spring and prevents the follower from tilting.
One issue of cramming 8 rounds in a tube that was designed to hold 7 rounds is that when a fully loaded mag is inserted in a gun while the slide is forward, that mag is fully compressed and the 8th round doesn't have much wiggle room and that results in the mag being harder to seat in the gun. This is the reason I'm switching from the Wilson 47Ds to the ETMs, longer tube allows for easier seating.
The longer tube of the Tripp Research Cobra mag is thought to be an improvement as it is meant to be an 8 round mag from the get go and will compress allowing for proper tension when seating the mag and still have a full skirt on the follower, not to mention that the steel reinforced polymer follower is pretty damn cool.
Mag tubes-Top: Flush fitting Springfield 7rd mag--Center: Wilson 47D -- Bottom Tripp Cobra Mag 8rd
Above shows the difference in tube length, it should be noted that the Wilson is a little longer than the flush fitting mag as it has to be to allow for the removable base pad.
So here's where all this starts mattering to this thread.
jdeere9750 asked why would anyone want a mag that holds less and I mentioned that they want a flush fitting mag, here's why.
Top: Kimber CDP Ultra with Tripp Cobra Mag 7rd -- Bottom: 5" Springfield with Wilson 8rd 47D Low Profile Base Pad.
In the picture above you can see that having that 7round mag now makes the grip area of a compact 1911 about the same same size as a full frame 1911 holding 8 rounds.
Top: Kimber CDP Ultra with Tripp Cobra Mag 7rd -- Bottom: 5" Springfield with flush fitting factory mag.
Above it's kind of hard to tell which is "bigger" in terms of grip area, but it's pretty darn close.
Here's some more shots of the the Tripp mag inserted in the Springfield
And some shots of the flush fitting mag
For those interested as it really doesn't apply to this thread, here is the Wilson with the Low Profile base pad.
I've never noticed any additional printing from having a longer mag, but some people worry about it which is why they favor a flush fitting mag for carry and prefer the longer mags for reloads.