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Erina's ass is simply huge. Absolutely enormous. Inconceivably massive. It's gigantic. Immense. Humongous. Its girth cannot be ignored. Chairs tremble in its mere presence. Couches cower in fear. It has its own ZIP code. Erina is legally required to have working brake, reverse, hazard, fog and indicator lights fitted on it. She is only able to leave her home through the garage door. Weather patterns are thrown into chaos whenever she goes outside. Bodies of water are completely displaced when she enters them. Earthquake warnings are set off across the Malay Archipelago whenever she sits down. Its size is so great that dozens of innocent people die under it weekly without Erina's knowledge, their remains never found. It singlehandedly caused tigers to be reclassified as the largest organisms to have ever lived. When asked about Erina Makina in an interview, Pierce Brosnan reportedly said "ass almost too big".
TL;DR it's sizeable.