What can I do to get my Hydrangea to be big?

hum what have you done?

To get bigger flowers, cut them all the way back in late winter or early spring. Hydrangeas will produce much larger blooms if pruned hard like this each year,but depending on plant variety this can lead to a floppy looking Hydrangea. Many gardeners opt for smaller blooms on sturdier stems.

You mean the blooms? there are different varieties like the globes .... https://www.google.com/search?client=opera&q=globe+hydrangeas&sourceid=opera&ie=UTF-8&oe=UTF-8

Cut the branches back they need stressed now is a good time

Pour some orange juice on it. Or fertilize.