So here's an interesting development. Heroes of the Storm's ranked play option is limiting party size to two people.
In a mid-August patch the party size for Hero League was set at a maximum of four (you could actually play as a full five stack but it would put you into the Team League queue to play exclusively against other groups of five instead of mixed opponent parties). After listening to the feedback and seeing how the patch played for a little while, the maximum party size has been shrunk again. Here's why:
"When we look at the data for parties of three and four players in Hero League, we often see large skill gaps among party members. This is fine for players in a party of four, for example, who have agreed to queue together despite any differences in skill that may exist. However, the matchmaker needs to complete the team by pulling in a fifth player who queued up alone, and did not necessarily want to play with teammates of varying skill levels.
"This can also create situations in which party members will discuss strategy with each other, but don't end up communicating the game plan to their fifth teammate. On the other end of the spectrum, solo players may disagree with the party's decisions, or show unwillingness to help the team. This can result in a friction for both sides if communication doesn't improve.
"What's more, parties of three and four players appear in Hero League queues far less often than solo players and parties of two. As such we feel that large parties represent a small enough portion of Ranked games that we're comfortable removing them from Hero League in order to improve overall match quality."
As a point of fact, Dota 2 acknowledges the problems of the four stack plus a lone player scenario by preventing parties of four people from queuing for ranked games.
The second paragraph of Blizzard's explanation, that it can cause friction, is the one I've seen manifest most often. Not in HotS, because I don't play that much and when I do it tends to be as a five or a three in unranked. But in Dota sometimes I will play unranked as a four stack if that's the number of friends available. We jokingly refer to the spare spot we haven't managed to fill as the "blame space".
I want to stress that this is entirely jokingly and we never make that extra person feel unwelcome but amongst ourselves on voice chat it can be cathartic to pretend everything that went badly was none of our faults. Miss a skillshot? Clearly player 5 was distracting you with their silence and competent play. Forget to buy wards? Obviously player 5 should have used his or her carry gold to pick them up. Game crashes? PLAYER 5 WHY ARE YOU HEXING MY PC?
But despite the joke there's a real disconnect in that scenario - four people on voice constantly updating one another on their plans and one person needing to use what information is available and fill in the blanks for themselves. I've been on the receiving end of that setup too as a solo player - some games it works out, some games you get Mean Girled. I can definitely understand why you would excise it from ranked play where people can get really antsy about YOUR TERRIBLE IMPACT on THEIR PERFECT SCORE.
The change will go live with the next game update and Blizzard say they're interested to get more feedback once people have had a go with the changes in place. I wonder if they'll settle on this or compromise on parties of three? I get that fewer people queue as a three but I do like the neatness of putting a two and a three together to form a team instead of faffing about with remainders. Maybe that's why I'm not in charge of the party size strategy.