ECCLESTONE SISTERS Mine is BIGGER Than Yours!!!

It's the female version of a genital challenge ... two wildly rich sisters who are vying for the title of L.A. land baroness ... and in this contest Tamara Ecclestone is giving little sis Petra a run for her money.

Real estate sources tell TMZ ... Tamara is on the hunt for a mansion more expensive than the one Petra bought from Candy Spelling which came with an $85 million price tag.

We've learned the not-to-be-outdone Tamara went shopping Monday with uber-realtor Mauricio Umansky (Kyle Richards' hubby) and spent an hour at the legendary Walt Disney mansion in Holmby Hills Monday -- up the street from where Michael Jackson died.

Price tag -- $95 MILLION for the mouse house.

We're told Tamara really liked the 35,000 square foot mansion with insane grounds, 8 bedrooms and 17 bathrooms.  

The two very competitive sisters have money to burn. Their papa is Formula One mogul Bernie Ecclestone, who's worth just shy of $4 billion.

We couldn't reach Tamara or Mauricio for comment ... or Walt Disney.