Well, you have a mass/spring/damper system tied to a force generating motor. One can design any part of this system in different ways to tailor the sound and presentation towards certain goals.
Simply put, the GR07 sounds as if the motor is of a low power design. It may be better to day it was designed in a way that doesn't offer really high power. At low volumes, it sounds fine. As you turn up the volume, the presentation plateaus. The earphone can get louder, but power and authority don't scale up along with it. The low powered motor struggles to move the diaphragm as quickly as needed. The sense of power, punch, authority, etc. soften and lessen. This is what the GR07 does.
We can experience similar traits when under powering a speaker, however the GR07 doesn't really respond at all to more wattage. I have Meier 2Move, and the GR07 doesn't benefit at all from the amp versus simply playing through a portable audio player like my phone which is interestingly a pretty weak output device. Some earphones scale significantly. Others do not. When they do not, it tells you that any limit of presentation is inherent in the design rather than your supporting hardware.
It may be wrong to call this a limitation though. Really it comes down to intent. VSONIC had goals for the GR07, and it should have been developed pretty specifically to those goals. I can't say why the design is what it is, but I would certainly guess it was done on purpose. There are certain benefits associated with the design that can be seen as desirable. Really, any design is just a collection trade-offs. I know VSONIC spent a good amount of time developing this product, so I'm sure they designed it into what it is now with purpose. We as end users may have our own personal preferences that say "I would have done this different," but that's just one person's preference. A company has to attempt to fit one product to the preferences of thousands. It's mostly a best fit approach, just getting close enough to make the most happy and the fewest disappointed. I could say I personally wanted a stronger motor. However, the side-effects of a weaker motor are desirable too. For example, the earphone is highly revealing, but because the motor isn't all powerful it doesn't make you cringe playing every little defect. This design choice actually allows a rather accurate earphone to come across more pleasant than it should be. That is a trade-off that many may prefer.