Yeah, people with epilepsy can take a higher dose of lamictal to help with their seizures (e.g., 800mg), but anything beyond 400mg usually does nothing for bipolar mood stability and anything beyond 500mg has diminishing returns for people with seizures.

The other thing is that lamictal can be toxic at high doses (i.e., anything greater than 400mg). One guy died from 4000mg of lamictal.

Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5167082/