I always play extremely well in small sided games - 3v3, 4v4, 5v5, etc, on smaller fields. I don't know why, but I just am always able to find a teammate, know where the players around me are, find through balls, shield and dribble effectively, tackle, etc. For some reason whenever I play 7v7 or 11v11 on bigger fields, I can't play this way. I feel like I play a lot slower, my decision making is always rushed and sometimes poor, as a result I have less confidence too. Is there any stuff I can do to change this? I don't understand how I can always be the best player on the field in the smaller games and then be so average/below my capabilities when the game is on a larger field with more players.
11v11 is more confusing, has a bigger space which means the timing and distance of the runs is different, and involves team tactics. The 11v11 team should be using a system of play which gives you an organized framework to work with your team mates. The normal learning progression is to understand the team's system of play and what the role of your position is. This allows the team to play as if the 11 players shared one brain. If you are a relatively new player, it takes some time before you learn to "read" the 11v11 game as well as you "read" small sided games. If you are an experienced player, the confusion may be due to your team not having an organized system of play. In that case their play would confuse an experienced player because there is no system to understand.
There is more going on in an 11 v 11 game. Since there are more players and more space, there are more decisions to make. Like rca2 said, a system of play basically narrows down that number of decisions. Frankly, the skill to make decisions quickly and not only that but to read the game and "know" where that pass will be going or another player will be running before they do so takes lots of time in the game. With your team, part of it might be team chemistry and time together...so that you know what your teammates tendencies are and which runs they are likely to make. You can focus on that (teammates tendencies) even if you are on the bench. You develop the skill of reading the game primarily by playing the game but also by asking lots of questions from your coach or other tactically minded people around you. If you watch pro games, I would suggest sometimes watch those games as a player and analyze their play (not all the time! its fun to watch as a fan sometimes!). Pick out one player and focus on the game decisions that they make, which runs they make, who they pass to and when, etc. Or watch the game like a coach would and break down the plays...what went wrong at certain points or what went right when they scored. Beginning to look at the game from that perspective will make you more aware of the game decisions you make and after lots of time playing with that awareness, it will become more natural.
Preparation before the ball is at your feet is the key. In coaching courses they have this slide that shows the number of decisions depending on the size of the game from 1v1 all the way to 11v11. Once you get above 4v4, it becomes extremely complex. I don't know if you are a chess player, but in chess every move is available to you HOWEVER most of the available moves are junk and only a few (or one) is the right one. Soccer is the same way, most of your options will be "junk" so you just have to, through experience, learn what options aren't even worth considering. Back to preparation. Before the ball arrives always be scanning in every direction and create a mental map of space, teammates, opponents, boundaries, and the goal. Then TURN YOUR BRAIN OFF. Don't think too much about "which decision should I make?", rather let your brain serve up ideas to you. Have you ever had those moments during play, I hope you have, where there is a quick little thought or voice that tells you to do something? That's your training talking—don't talk over it by doing "active thinking". You train to make your actions automatic. If you can, go and play 11v11, against weaker competition as a form of training. It will allow you to play and have a bit more time to make decisions.