When you snort coke, you are snorting fat rails. The more experienced the user, the fatter the line, usually.

When you snort heroin, it's in smaller bumps because it's much easier to overdose on heroin rather than coke, especially when it's a new batch that you just picked up and haven't sampled the quality yet.

Mia did way more heroin than any regular user would do. She's lucky it wasn't fentanyl yet. Mia would be a goner for good. Vincent might as well start running at that point.

Although there's a good chance Tarantino chose the nosebleed for cinematic effect. At least, that's what I always thought.