It looks like you have stumbled upon the world of "diminutives". A diminutive is a form of a word that modifies the original, the purpose being to transform it into a, well, diminutive form. These forms often indicate familiarity, but it can also indicate that it is the "smaller" or "minor" version of something.

There is a thorough book on the subject entitled Diminutives In English by Klaus P. Schneider. From page 2 of the book:

That English has no diminutives is a common myth. The truth is that English does have diminutives. Yet, due to the general structure of the English language, English diminutives are primarily formed analytically by using little. Additionally, however, English possesses an inventory of diminutive suffixes, such as, e.g.,. -ie, -ette, -let, -kin, -een, -s, -er, -poo, and -pegs, for forming synthetic diminutives.

On pages 76-78, the author gives a more thorough list of 86 diminutive suffixes in use in the English language, a couple of them being -a and -ia.

English is something of a garbage disposal of a language, and many word forms were originally borrowed from languages outside of English's Germanic roots. The -a diminutive suffix is not native to the English language, but as you suspected, is more common in the Romance languages. There may be a Slavic root in some cases as well. In addition, many of the -a names were borrowed into a continental Germanic language (such as German) first before coming to English. For instance, the English language historically uses the name Catherine, while German will use a form such as Katharina. English will use the form Mary, while German, Scandinavian, and Romance languages will often use Maria.

Instead of the -a diminutive, English prefers to use the -y or -ie diminutive form with male given names. John becomes Johnny, William becomes Willie/Billy, George becomes Georgie, Harold becomes Harry, Robert becomes Bobby/Robbie, and so on.

This also applies to female given names, too, where Anne becomes Annie, Elizabeth becomes Lizzy, Catherine becomes Cathy, etc.

However, when using a diminutive directly as the given name in English, then it is much more common with female names than male names to use that borrowed -a suffix: Anna, Eva, Elena, Lisa. This is often the case when forming a female name from a male name: Roberta, Olivia, Carla, Paula, Victoria. Though -y is also common: Emily from Emile, Lucy from Lucas/Lucius, Stephanie from Stephen, etc.

When male names in English end in -a, then, usually, it's not directly a diminutive form. In most of the popular male -a names, they're direct borrowings of Biblical names as they appear in the Latin Vulgate Bible: Joshua, Isaiah, Jeremiah, Jonah, Micah, Jedediah, Ezra, Zachariah. Some, of course, have only ever had a low level of popularity, while others have been Anglicized into the more familiar -y/-ie diminutive, particularly Jeremy and Zachary.

But in general, until recently, at least, it was considered impolite in the English language to call a full grown man by his diminutive form, unless you were a close family member or a childhood friend (and even then it wasn't always polite, depending on the feelings of the person). Or, as Schneider explains on page 148:

Using (IE)-dimunitive to address adults violates the polite norm, at least when used for males. In particularly close relationships, however, such forms can be used to express affection, or to treat the addressee like a little child, e.g., in social games such as kidding, teasing, chaffing, bantering, etc.

Schneider goes on to point out that the opposite happens between parent and young child: a boy who is usually addressed as Stevie will understand the annoyance or impatience of his parents when they call him by his full name Stephen.

More generally for adult males in the English language, the more historically acceptable form of showing familiar, diminutive form is to use the "short form" of the name. Stephen doesn't become Stevie, but becomes Steve. William becomes Will/Bill instead of Willie/Billy, James becomes Jim instead of Jimmy, Robert become Rob/Bob instead of Robbie/Bobby, etc.

In short, male -a names aren't common in the English language because the borrowed diminutive form became identified with females instead. The female name, in fact, is often a transformation from a male name by adding that -a. The more gender-neutral diminutive in English is the -y/-ie form. Because of this, -a names, historically, were often considered to be "girl's names" in English.

A male immigrant to an English-speaking country would often choose to Anglicize his -a name to its English equivalent, to avoid confusion, teasing, etc. So Nikola elected to adopt the name Nicholas/Nick in his new country, Alexa became Alexander/Alex, Stanislaw became Stanley, Sasha became, um, Sam, etc.