Having less players is always worse. In my bracket/region, usually the team that has a leaver simply leaves the game as soon as it’s safe to leave, since they’re almost guaranteed to lose playing 4v5. Only exception is if the game was already going very long or the leaver team had a clear advantage.

Let’s talk about the real impact a leaver has: around 20 minutes, the top cores maybe have 10k networth. For ease of calculation, let’s say the leaver has all of that in items. That means when you sell their items, your team gets 5k gold distributed among the remaining players. That’s 1250 gold per hero. Sure, those 4 players may now be a bit stronger. But also, the whole team just lost 5k gold instantly, because you only got half the money when selling items. A 5k networth swing at 20 minutes alone is already pretty bad. But you also lost a whole hero. And this also gets worse over time, since the hero doesn’t earn any gold except passive gold anymore, which is so small to be negligible (as was already said, it’s less than 2 last hits per minute, which is laughable).

So just viewed objectively from the state of the game, having a leaver is terrible. There is no gameplay reason that a team with a leaver should win more often than not, in fact it’s such a huge disadvantage that it’s pretty normal for the leaver team to just stop playing.

So there are two things to check here: One is, do you really consistently lose against teams with a leaver? How often does that really happen? How many leavers do you get? I have one maaaybe every 20-50 games, so every 15-35 hours, even if I theoretically would lose against leavers more often, there is no way I would notice that because it just happens so rarely. Did you not just maybe randomly lose 2, 3 games in a row against leavers and extrapolate from such a small sample size? That could just be random events because the enemy team already was really far ahead in these games, so while having a leaver is devastating, they were already so far ahead that it didn’t matter too much.

The second one is mindset, for example it’s not very uncommon as the team without a leaver, to “stop trying” as hard and mess around a bit, because “the win is basically guaranteed”… but if you do that during the late game without a buyback, it’s really easy to still lose the game. Even though you have a huge advantage, if you just play proportionally worse, of course you can also still lose.

The only way to really help you though is for you to give us the match IDs in question and let us take a look at the games that you lost with a leaver. Without that, everything is speculation.