Margot Robbie Sporting Big Baby Bump for 'Once Upon a Time in Hollywood'

Margot Robbie's blowing up, particularly around her belly ... but only to play the tragic role of Sharon Tate.

Margot was rocking a prosthetic baby bump Tuesday while filming scenes for Quentin Tarantino's "Once Upon a Time in Hollywood" -- set, of course, in 1969. Note the classic cars behind her for the scene.

Margot's transformation into Sharon is uncanny ... and the sight of her "pregnant" is a stark reminder of Charles Manson's brutality. Sharon was 8 1/2 months pregnant with Roman Polanski's son the night the Manson family members murdered her.

Sharon's sister, Debra, told us she's thrilled with the casting, and thinks Margo will fill the role in spectacular fashion. It certainly looks like she's nailing it.

Hard to imagine, but Brad and Leo might be getting upstaged.