That's a good question, young acolyte. One that mages have been asking ever since Magister Lanaken first formalized the dimensional storage techniques. For a full discussion on the thaumaturgic explanation I'll direct you to Kenshian's 'Dimensional Theory' - third edition, by preference.
Briefly however, the apparently unlimited space accessible to us through dimensional storage magic is not the same as the world we experience. Broadly speaking the rules that we work by in our daily lives have no relevance to the storage spaces. Size, mass, speed, pressure... none of them have the same meaning within the storage space as they do in the natural world.
According to Kenshian it may be incorrect to call it a space at all. In her model she treats these storage dimensions as conceptual regions rather than physical ones. This certainly explains a number of things that have befuddled mages much more experienced that you or I. Let's run through some of the common questions shall we?
Why are storage spaces so large?
Simply put, they're not. Size does not exist in a dimensional store, so it is neither large nor small. The aperture, the connection between your storage space and our world, is a thaumic construct which performs a simple translation between the two. As you pass an object through the aperture it assumes the properties of objects in the storage space. If Kenshian's model is correct then objects are translated to conceptual objects, without any physical properties. They occupy no space, have no location, no mass.
Considering the sheer amount of stuff my own storage space contains, I'm rather glad that is the case.
How do we access objects when we need them?
Just like the aperture transforms the properties of objects, it also transforms your will through the same mechanism. When you reach into the space to remove a potion for instance, your will is impressed on the conceptual space. The concept of your hand, guided by your will, simply grasps the concept of the potion. If no potion concept exists within the space then you'll fail. If your will is diffuse then it might grab the wrong potion. If I have fifty different potions in my store and I just reach for "a potion" then what I withdraw is, apparently, random. The better you define the specific object the more likely you are to retrieve that one item and not something similar.
I think we'll all agree that this is a fabulous boon compared to the older spatial storage techniques, where one would have to fumble around among the physical objects.
But we can't transform artifacts, how can they be stored?
Seems like a real problem, but when it comes to conceptual spaces it might not actually be relevant. The rules of a conceptual space simply apply to everything that enters them. We speak of the aperture as having a transformative effect on objects, but in reality it's just acting as an interface between the conceptual and physical. No physical thing can exist in a conceptual space, so any physical object - no matter how empowered, regardless of form - can only have conceptual properties within the space.
Gruent's treatise "Metaphysical Representations in the Realms" covers this in greater detail, with broader discussion on other types of spaces such as the so-called elemental planes.
If they're just concepts, why do they stay the same?
Honestly, Gruent does a much better job of explaining this. It's not my field.
The short form is that there's something that defines the object, and that something doesn't change just because the object moves to a conceptual space, an elemental space or a physical space. Gruent calls this the ontic form of the object. Moving between spaces doesn't alter the ontic form, only how that form is expressed within the rules of the object's reality - the space it occupies.
Why don't things cool or spoil when I store them for a long time?
Temperature, corruption, time itself? Your storage spaces contain none of these things. Not as we mean them anyway. Clearly there's something like time, since we can change the contents of the storage. But while there, objects don't experience the passage of time.
And before you ask, no, we can't store people. Or complex animals. Conceptual spaces appear to be compatible with the spiritual essence that distinguishes living creatures from dead ones. Passing into a conceptual space damages the spirit in direct proportion to the complexity of the spirit. An insect may pass into the space and emerge apparently unharmed, but it dies soon after. A cat emerges insane and dying. The last time a human trial was conducted the results were horrific enough to dissuade further experimentation.
Any creature with will is affected, except those whose spirit is attuned to certain esoteric spaces - some elementals for instance and most divine entities, although I don't suggest you try this without further study. Ensouled items are also unaffected, and report no experience within storage, as though they were immediately withdrawn regardless of how long they are stored.
And that's the end of today's session. Next week we'll cover some of the important caveats of using conceptual spaces, their limitations and the few non-living exceptions. I expect all of you to submit a medium essay - that's three full pages, Mr Druvek, without illustrations this time please - on the formation of storage spaces and maintenance of their physical anchors. With references to DuVrey, Ak'then'kar and Malcolm.