Are you talking about hardy Geranium species? The commercially available Geranium species have mostly violet to reddish purple, magenta, and occasionally pink or white blooms depending on the species. While some have a lot of blooms, they don't resemble balls to me.

Humidity keeps temperatures from falling as much overnight. When the low temperature never falls below 80º some plants struggle. More common in the southern states are the Pelargonums. These are mostly from South Africa and are only reliably hardy where freezes are rare, light and short. Nonetheless, my mother wintered over her pelargonums on her porch in zone 8b.

Yes, pelargonium. I am talking about these:

Pelargoniums

I love the red ones!

Can I ever hope to make them look like these below?

They are literal red balls! :-)

I never cared about hardiness because I grow them in pots as annuals anyway.

I am afraid they have either not been getting enough hours of direct sun or my railing containers are too shallow for these plants to develop a proper root system.

I look forward to trying them out next summer, in the front of the house, in Western sun and deeper pots.

Those railing containers always look shallow to me too. Perhaps you should take cuttings now to grow through winter for new plants next year in larger pots. In general, pelargonums prefer slightly acid, well-drained soil, and regular but light fertilization every 2 weeks. Look for a fertizer that provides micronutrients like magnesium for best results. They do like at least 6 hours of sun, but should be shaded in the afternoon in areas with temperatures above 90ºF. HERE is an article with more info.