I am unable to run longer campaigns with "bigger" groups (4 to 5 people) since making agreements on playtime becomes very difficult then. People are overstretched on rpg sessions, lots of call offs at the last moment, etc....
However most of the time I find it easier to make agreements with smaller groups, most of the time groups of two people who are a gamer couple.
I find it very difficult to create balanced encounters for such a small group, while they are able to rp themselves out of trouble most of the time sometimes they just hit the blade.
Especially around CR5 monsters start to have special abilities which can render a party of two people completely helpless in a turn.
My question is, what do other gm's do to help such groups to have a surviving chance ?
Smaller groups allow you to pay more attention to every player and it's easier for everyone to work out complex plans without everyone talking at the same time. I actually prefer to keep groups at 4 players or fewer.
At the same time, fewer characters also tends to mean that you run out of spells faster and will likely rely a lot on potions and scrolls.
These two things combined make very practical to focus campaigns with smaller groups more on exploration and social interaction than on combat. Things are not as hectic and crowded, so you can take your time for finer details.
A good thing then on my current group (with the couple) I went for a home brewed campaign and not a commercial campaign since they are more combat focused.