Total coolant system holds ~5gal depending on model year and options. Tank obviously doesn't hold all that much. Sounds like you had air trapped in system and extreme heat caused a big burp. I've seen coolant evacuations from tank happen in both high heat and high attitudes. As a precaution, get it pressure tested. Otherwise, make sure you properly burp it while refilling or use a vacuum system to ensure no trapped air which could cause catastrophic hot spots...
Good luck
