An analogy for better understanding:
Lets say you want to send six 1 L wine bottles to a friend and you put them in a 10 L package. You don't want the bottles to break so you add 4 L of bubble wrap to the package. You know what is in the package and if I ask you, you would tell me that you are only sending 6 L of goods and that the package is only 60% full as you could open that package, throw away the bubble wrap and replace it with for example 4 more wine bottles.
The postal service doesn't care and will charge you for the full 10 L and not only for the 6 L of goods that are inside. They are not allowed to open your package and can't know what's inside. All they care about is that your package is 10 L in size, so it will waste 10 L of the delivery vehicle's trunk that can't be used anymore to transport other people's packages. So according to the postal service that's a 10 L delivery, even if you tell that them it's only 6 L of goods.
If you don't want to be charged too much don't put your goods in a way too big package. Choose a smaller package size so you don't need that many packaging materials to fill the unused space. You will be happy as you won't be charged that much and the postal service will be happy because they could fit way more 6 L than 10 L packages in the same trunk.
And your 6L of wine bottles in a 10 L package might consume even more than 10 L of trunk space. Because to secure the cargo, while transporting it, they might put your 10 L package in an 11L transport box. So, in the end, it's consuming 11 L of space.
The postal service is your PVE host. You are the guestOS. The package is your VMs RAM. The trunk is your PVE hosts RAM. The wine bottles are your guestOSs system+user processes and the bubble wrap is your guestOSs cache. The transport box is the virtualization overhead.
