Nah, it's not just California. California's allotment is almost 30% of the whole thing, which is larger than any other state's, but ⅔ of that goes to agriculture in the Imperial Valley, which ships produce all over the US and other countries. And being at the end of the line, California does benefit from surpluses, although those are drying up, which is why the coastal cities from LA to San Diego are doing their damnedest to grab that ag water for themselves.
Really, the whole Colorado River allotment system is a mess and because there are 7 states (and Mexico) involved, all the competing self-interests make it really hard to fix.
You've got farmers in the upper basin who are incentivized by use-it-or-lose-it rules to flood fields with water they don't need to protect their allotments. Utah is trying to divert an immense chunk to the fast-growing St. George area to incentivize the growth without responsible conservation efforts. And then other factors play into it, like Arizona farmers who grow cotton with way higher water use and less demand than other crops but is supported by farm subsidies which make it a better money move. And so on.
It's all ugly. Historically, I'd say Mexico has gotten screwed more than anyone.