The wording around Vhagar's size compared to Balerion changed in later books

There is an interesting difference between the way Vhagar's size is described in earlier and later material.

 

In The World of Ice & Fire, the comparison is more specific: Vhagar is described as being nearly as large as Balerion was at the time of the Conquest.

 

That is an important qualifier. Balerion during the Conquest was already enormous, but he was also only around 120 years old. He lived for another 70+ years and continued growing until his death.

By the time of the Dance, Balerion had been dead for decades. Vhagar was around 181 years old, and other dragons show that growth slows dramatically but does not necessarily stop until very old age, alos stated by Fire and Blood.

 

Then later sources change the wording:

 

Fire & Blood describes Vhagar as nearly as large as “the Black Dread of old.”

 Though slower than she had been a century before, she had grown nigh as large as The Black Dread of old. Her fires burned hot enough to melt stone, and neither Caraxes nor Sheepstealer could match her ferocity.

The Rise of the Dragon, also encyclopaedic and written by the authors of the world of Ice and Fire, says Vhagar was nearly as large as “Balerion the Black Dread.”

 

Rise of the Dragon, the statement

The difference is that the earlier statement gives a specific point in Balerion's life: Balerion at the Conquest.

 

The later statements simply refer to Balerion the Black Dread, the legendary dragon as a whole.

 

Normally, when someone says “X was nearly as large as Balerion the Black Dread,” the natural interpretation is not “Balerion when he was younger.” It is the famous Balerion people remember — the dragon at his greatest size.

 

For example, if someone says “he was taller than Alexander the Great,” nobody assumes they mean Alexander as a child. They mean Alexander as the historical figure.

 

So this is a progressive shift:

 

A World of Ice & Fire: Vhagar ≈ Conquest-era Balerion.

Fire & Blood: Vhagar ≈ “the Black Dread of old.”

The Rise of the Dragon: Vhagar ≈ Balerion the Black Dread.

 

So, the wording of all recent sources does appear to make Vhagar's implied size close to Balerion at his legendary peak rather than specifically Balerion during the Conquest.