Yes, do consider keeping them in a slightly larger pot. I just received yesterday a Lil Honey Oakleaf hydrangea in a 10cm pot from a place called Hirt's in Ohio. The shrub looks like it was cut dwn to minimize shipping weight but the main stems look quite thick. I should be moving it to a bigger pot soon.
In hot Texas, I tend n-o-t to plant anything outside starting in May because it gets uncomfortable starting at 10am. I also try not to buy anything until temps moderate so I do not have to mess with watering it, etc. But you know, sometimes one does find a good sale item or an unexpected item.
I will be moving the hydrangea to a 1 gallon pot (3.8 liters) and keeping it in dappled sun or in morning sun/afternoon shade conditions.
Oakleaf and paniculata varieties can grow quite vigorously and I will plant them in the Fall if they quickly grow a lot but, I will wait until temperatures moderate here in September. So that I do not forget, I will put an electronic reminder on my computer to remind me every weekend in September that I need to consider planting the hydrangea at that time... if it looks large and ready... and if the winter weather forecasts are out.
Mopheads, lacecaps and serratas do not grow as fast and will be planted either in the Fall or more tyically in the Spring... again, depending on how tall they get, depending on how ready their permanent location is, depending on long term weather forecasts (is winter going to be unusually cold or dry), etc.
If winter will require watering because it will be dry or if temps will be a lot colder than normal, I may keep them in the garage and plant them outside a couple of weeks after my average date of last frost in Spring. But our last few winters have been unusually mild even for here and leaf out has begun as early as January so, there is no telling if I will need to adjust plans.
I also have dogs that can damage these small shrubs/pots when they are chasing squirrels... or they looove to eat some of the organic fertilizer that I spread on plants.
So... better to put these small shrubs temporarily where the pooches will not be able to stomp on them or try to eat try to eat every tiny sliver of the fertilizer in the pots.