A post from Battle.net community manager Spyrian outlines the latest changes Blizzard has implemented to matchmaking in Heroes of the Storm. This is intended to address complaints that pre-made teams were sometime being matched against groups of five strangers, leading to imbalances in communication and build coordination between the teams. As of the most recent changes, this will no longer happen, as they have "greatly reduced" cases where one team has a party with three participants to prevent solo players from be matched against four or five-player parties and duo-queues from be matched against five-player parties. Here are more details:

As part of our ongoing improvements for the Heroes of the Storm matchmaking system, we’ve just implemented a new change that will affect what types of parties you may get matched against while playing Quick Match. Starting today, we’ve greatly reduced the possibility that you’ll be matched against a party that has three more members than the largest party on your own team.
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Let’s expand on what that means. Take a look at a few common party sizes below, and what you can expect to see in Quick Match after today:

• Solo players: Cannot be matched against four or five-player parties.
• Duo players*: Cannot be matched against five-player parties.
• Three-player parties: Can match against any party size.

It is important to note that the matchmaker will still prioritize match quality over party size, and may pull in parties outside of these ranges if the resulting game is the most suitable match available. This means that it’s still possible to play against a party that’s larger than your own, but this will only occur in rare cases.