EXCLUSIVE

Evidence Octomom scammed tax payers is growing faster than Lauren Silverman's belly ... after authorities turned up more proof of possible welfare fraud ... thanks to pay stubs from her masturbation porn.

TMZ broke the story ... the L.A. County of Welfare and Fraud Dept opened an investigation against Octo -- seizing her financial records -- and determined she'd been overpaid $15,683 in welfare funds from Jan to May of 2013.

Now the evidence has grown ... authorities seized payment records from Wicked -- the company that hawked Octo's masturbation porn -- and uncovered Octo's hefty XXX residuals:

January -- $1,839.74

February -- $2,537.27

March -- $1,807.97

April -- $7,381.99 (Easter gifts?)

May -- $3,026.67

June -- $1,295.37

6-month total: $17,889.01 (Again, these are residuals).

The discovery adds fuel to the fire ... that Octo is a welfare cheat.