Losing weight will only make your penis look dramatically bigger if you're at least 30 pounds overweight because the pad of extra fat over your pubic bone hiding your penis will get smaller as you lose weight to reveal your bigger penis.
You can gain an extra inch in penis size just by losing weight if you're already at least 30+ pounds overweight but overall, Losing weight will make your penis look bigger by increasing how proportionate your penis appears to the rest of your body.