Exactly, what you are seeing is that the LiHV packs are just in fact overcharged standard cells. Lets imagine a standard 4.20VPC 2S 4000mAH pack. This pack instantly transforms into a 4.35VPC LiHV 2S 4500-ish-mAH pack when overcharged to LiHV voltage. Same exact LiPo with same exact dimensions. The only difference is the cells in the LiHV version are high enough quality to be overcharged without catastrophic failure. The end, thats how LiHV is made. In order to gain capacity WITHOUT changing a given voltage, a battery MUST grow in size physically.

Basically what has happened is because LiPo has been around for about 2 decades at this point, the technology has progressed to where the highest quality cells can be SAFELY subjected to overcharging, its really that simple. The only difference between now and a few years ago is that the cells weren't as strong and so even though you could have invented LiHV yourself by simply overcharging your standard LiPo, it would more than likely result in catastrophic failure and an explosion of fire. So really, any one of us could have been the founding father of LiHV. Ironic, isn't it?