It's just because i know that the recommended maximum volume for an IM depot is 2ml. However those recommendations are guidelines set by Substance Misuse Services in the UK and are likely a conservative estimate... for example, a 130lb junkie would not be able to handle as much of a depot of any drug (in someone who's veins are shot it is common to IM a rec. drug rather than IV) as a 220lb bodybuilder.
But yeah.. i stick to 2ml or under. If i have a 3ml injection i split it in two.
I get free pins plus it takes no time to do and i am quite experienced with injecting so don't ever give it a second thought. Eyes closed scenario..
Someone with no free or limited access to works or someone who is nervous about the procedure (which can take a lot of time to 'get used to' - especially when cycling vs. injecting daily) may be tempted to do one larger injection over two smaller ones, paradoxically though this also means they will have had less experience so may be more likely to experience pain at the site...
I stick to 2ml max and suggest that to people i talk to - but it isn't wrong to do otherwise AFAIK.