What actually makes a lungo bigger and more bitter?

A lungo is bigger and more bitter because more water is pushed through the same dose of coffee grounds, extending the extraction time and volume compared to a standard espresso. As more water passes through, it continues pulling compounds out of the grounds beyond the ideal extraction point, including bitter and astringent compounds that a shorter, more concentrated espresso shot leaves behind.

The bean roast level isn't what changes between an espresso and a lungo pulled from the same beans — they can use identical beans and identical roast. Milk isn't part of the comparison at all, since both espresso and lungo are typically black shots without milk added.

This is why baristas describe over-extraction as pulling too much from the grounds: past a certain point, the water starts extracting harsher plant compounds rather than the pleasant acids and sugars, which is exactly the tradeoff a lungo makes for its larger volume.