You should give plenty of room to the large vehicles in the roundabouts because:

• They have to swing wide within the roundabout.
  Explanation:

  "

  You must allow extra room alongside large vehicles (trucks and buses) because they may have to swing wide on the approach or within the roundabout.

  "
• They are fast-moving vehicles.
• They cause less turbulence.
• They have small blind spots.