Stunted growth? Please Help!

Well I'm confused, most of my plants are about a foot to a foot and a half tall, and haven't shown much growth in the past week. However, one of them is about 3 feet tall, and looking great. The only difference between them is the amount of soil in the containers. The plant that is looking huge and doing great was the last plant potted, I was low on soil so only ended up filling it to about 2/3rds - where as the other containers are filled nearly to the top - I was told that the soil would settle with time. Well they havent settled, and since I made a rookie mistake in using miracle grow organic, they have pretty poor drainage, and yes you guessed it; gnats. So now I feel like Lebron James, "What should I do?"

Can over filling containers with soil cause problems? Or could it just be that the root ball was able to fill the container more quickly on the one big plant I have, and therefore able to grow more efficiently? In that case the other plants should catch up to it eventually when the root ball fills the containers, right? In my "stunted" plants the soil seems alot more compact than in the "healthy" looking one, and with the steady rain I'm not sure any of them have dried out completely in between waterings yet. Is there anything I can do besides hope for the best? and on a seperate issue would anyone suggest using insecticides? Can ants hurt your plants?

If anyone actually reads this and knows what they are talking about any advice would be helpful, if any more info is needed to diagnose or help me than just ask. Thanks. Mainly I'm worried if the soil I have could be too compact, and if theres anything I can do about it. Unless you guys can think of other reasons why I might have 1 plant be so much bigger than the rest when they are the same strain.

Friend i grew 5 i little while back not the best attempt but had 4 in smaller pots thanthe the one that out grew them all, i av allso read that bigger the container the bigger the plant, it happened to me now it looks like this is the case with u, 25% more soil would probly mean 25% more growth, just my experience, but see wat others chip in, good look man !!!!