Umm, well, you would need much larger pre-comps to accommodate the glow halo. This has nothing to do with the layers or the parent comp itself.
Thanks for your reply. I'm afraid I'm new to AE. Can you explain what I would need to do then, or point me in the direction of a tutorial?
Btw my Pre Comps are 1920x1080, as is the main Composition.
I'll say that "Flare3" nested comp is scaled down to... oh, about 25% and it's been repositioned. To get no hard edges on the flare, you should make that comp at least 6 times larger than it is now. So 1920x6=11520 and 1080x6=6480. That's how big it ought to be. Perhaps bigger, depending on what you're doing with it.
I have to differ with Dave and Mylenium on this problem. You have kind of a Catch 22 with the standard Lens Flair. The flair size is calculated based on the layer size. Lens Flair will also not extend beyond the layer boundaries. If create a solid that is 5X the size of your comp and scale it down to 20% so that is will fit the comp and you apply lens flair, the size of the lens flair will be exactly the same as a lens flair applied do a solid that is the size of the comp and scaled to 100%. There is no work around that will solve this problem because the lens flair is calculated based on the size of the layer and the lens flair will not extend beyond the layer boundaries. The bigger the layer, the bigger the lens flair.
Comp 1 is the default lens flair applied to a layer 5 times the size of the comp and scaled to fit, Comp 2 is the default lens flair applied to a solid that is the same size as the comp and at 100%. They are the same size.
Grow bounds won't work with lens flair. Pre-composing or nesting a huge comp won't work either because the lens flair will not extend beyond the layer boundaries and if you make the layer bigger then the lens flair will get bigger.
The only real solution is to control the size of the lens flair with the brightness and control the position with the Flair Center. You need to rethink your approach or create your lens flair using a different technique.
Thank you all for your replies. Much to my disappointment Rick, you are totally right! Thanks all, I shall try an alternative solution.
Your advice is greatly appreciated.