Green’s Dictionary of Slang

1. (US) a fat person, usu. female.

c.1943		‘Bill O. Lading’ You Chirped a Chinful!! n.p.: Big Berthas: Hefty dames.
1951		P. Whelton Angels are Painted Fair 21: In the aperture stood a hefty mama. [...] This Big Bertha snapped, ‘Well?’.
1980		Maledicta IV:2 (Winter) 227: Dora (Dumb Dora) and Dumbo [...] are heard and so is Bertha for a big one (elephant or chubette, delight of the chubby-chasers).
1989		P. Munro Sl. U. 35: bertha overweight girl.
1997		K. Kainulainen ‘University Euphemisms in Calif. Today’ 🌐 Names can be used as euphemisms too. ‘Bertha’ and ‘judy’ are both used to describe overweight girls.

2. (US camp gay) a nickname for any tall, heavy-set man, esp. if effeminate.

1966		F. Elli Riot (1967) 179: ‘Careful, Bertha!’ [...] Big Bertha, the first string centre on the prison’s football team, a two-hundred-and-twenty-pound queen.
1972		B. Rodgers Queens’ Vernacular.
2003		K. Cage Gayle 57/1: Bertha adj. and n. big, large (man). [Ibid.] Big Bertha n. tall, heavy set man, especially if effeminate.