For example, cells of the rod-shaped Gram-positive model bacterium Bacillus subtilis ... the Gram stain) than their Gram-negative counterparts (20–35 nm for B. subtilis and 2–7 nm for ...

If the peptidoglycan layer is thin, it's classified as gram negative. A gram-positive bacillus doesn't have an outer cell wall beyond the peptidoglycan membrane. This makes it more absorbent.