I respectfullg disagree.
Lets look at the evidence, shall we?
a) What other 'Planet Sized' beings are we aware of? Akash'Bhuta. We know her size can vary - In Mists of time in the slice right before the entity, we have her face filling up the entire side of the planet. Likely she is projecting herself against whatever cosmic entites are facing the planet. She may be an enemy because she is Chaos Incarnate, but she is also part of the spirit of Earth, which makes sense that it would fight against cosmic agression
b) Akash size changes. Mists of time, she is as big as a planet. Moutainous Carapace, her eyes are as big as the heroes.( not as big as entire countries) Just Doin' My Job, she is smaller than Jim as he shoots her. Depending on circumstances her size changes - possibly in response to threats/damage.
so we have established that cosmic entites can change shape/size. Now lets look at the one on Mists of Time
1) It may be a stylized represnation of the being extending its influence to earth. It does not meant his is the beings actual size. If that were its actual size th ere would be nothing the heroes could do to fight it directly - it likely must shrink down in order to invade the planet itself.
2) Being stylized it seems that the entity is made up of a sort of misty outline filled with stars. There is no way to tell if it has the Symbol on its forehead or not. It simply doesn't have enough form. Should it shrink itself down to fight on the planet, ie in Reveal the Flaws, there is no reason to think that it might not get its symbol then.
Their is a third explination of course, in which we both are correct.
The smaller being on Reveal the Flaws is the avatar of the cosmic entity on Mists of time. In which some might call it a servant, but it is possible that it is just its planetary representation and the symbol is the connection to the remainder of the entity in the cosmos. This is, in my minds, the most likely scenario - it must have some kind of avatar to invade the planet for whatever sinister purposes. If it was just a servant, it would be more like Silver Surfer, and defeating it would have no relveance to defeating the main entity - which is bigger than entire planets. If it is an avatar, then defeating it would mean the entity must gather more strength before it can attempt to walk the planet again, because of whatever comic book restriction keeps it from simply crushing earth in the palm of its hand.