Ground beef patties shrink because the fat melts, or renders, out of the beef as it cooks. If you're using a high fat ground beef, like 75/25, your patties will shrink significantly because 25% of those patties are fat. I recommend using a lower fat beef, like 85/15 or 90/10, to ensure your patties shrink less. Keep a close eye on your beef and don't overcook it, because lower fat beef can dry out more quickly than a higher fat beef.