Though Mitt Romney made his reputation as a private equity investor, his newly released tax returns show the presidential candidate has a large portion of his wealth in an even more aggressive corner of the investment world: hedge funds. A Globe examination of Romney’s taxes and a prior financial disclosure shows that about one-third of the assets in his Individual Retirement Account are in hedge funds, predominantly those run by his former firm, Bain Capital in Boston. Romney’s IRA could be as large as $101 million, according to his financial disclosure form, although he is not required to provide the exact amount. Of the 25 investments in that retirement account, 11 are Bain hedge funds.