You say your wins would total maybe 400. Ask yourself, what would be your total loses? Lets say for argument’s sake it’s also about 400. That gives you roughly a 50% win-rate.
Your opponents have golden portraits. with over a thousand wins. Ask yourself, how many loses do they have? If they have 1.5k wins and 1.5k losses, their win-rate would also be about 50%.
In this hypothetical, if their average rank was rank 7 with a 50% win-rate and your average rank was rank 7 with a 50% win-rate, your MMR’s should be close to each other. The difference is how much the two of you played. You played 800 games, they played 3k games. You both averaged to rank 7 with a 50% winrate. Your skills are the same, despite the number of games you played.
Keep in mind, I don’t have the numbers. This is all hypothetical. Maybe the MMR IS wrong. There is the possibility that people are being evaluated incorrectly. There is also the possibility that the MMR is correct. The possibility that your skills are even, you just play far less often than your opponent does.