He can still keep up with you guys because he doesn't die as much as you do.
When the lights are overhead taking potshots he's cutting you all in half with his minigun.
When you're out of buffs and begin to lose hope... IS THAT A HEAVY?! WITH A SYRINGE!?
When you're feeling outnumbered and terrified. He'll be next to you like a big brother to help you defend that objective.
He's there when you need a friend to lean on, encouragement, or a huge badass with a squad automatic weapon.
He may not be able to jump to that ledge but his chaotic frenzy of bullets can reach it just fine.
He's too busy using his huge amount of buffs so that his team will become unstoppable to worry about going up there anyway.
Thank you heavy. We love you.