The paniculatas have been pruned because they will grow flowers on this year’s growth.  If you prune severely to the first 2 buds, you’ll get fewer very large heads.  If you prune to 5 buds you’ll get more but smaller heads.  No pruning at all and you’ll get lots of very small heads.  Where we have 2 plants of the same cultivar, we’ve experimented with pruning the front one to 5 buds and the back one hasn’t been pruned at all.   The first is Hydrangea paniculata ‘Vanille Fraise’ pruned to 5 buds.  The second image is the same plant but which hasn’t been pruned.

Hydrangea paniculata ‘Vanille Fraise’ pruned to 5 buds

Hydrangea paniculata ‘Vanille Fraise’ pruned to 2 buds