Many roundabouts have two lanes around them. Usually this is to allow traffic to go in multiple directions. Sometimes it also allows multiple lanes of traffic to proceed straight ahead. In either case, side-by-side positioning with another vehicle can lead to major problems.
Choosing the inside (tighter radius) lane can make you vulnerable to the ‘pinch’. Most drivers won’t hold the centre of a lane as they negotiate the roundabout and may cut across to smooth the trajectory. There is also the possibility of a driver deciding to turn right from the left lane (generally also missing a signal!). This driver will cut right across your path and may cause an accident. An articulated lorry is unlikely to hold their lane and the trailer will cut lanes even if the tractor unit does not.
As a general rule, it is best to be before or after another driver when joining a roundabout but not at the same time. Staggering so you both have clear space to the side will give you both options should you need them.