Last year, when EQT bought hearing aid maker Siemens Audiology Solutions from the German engineering group, the buyout firm made an effort to share the deal with some of its smaller investors.

The Swedish private equity firm called for 10 investors to put €10 million each into the €2.15 billion buyout, according to people familiar with the deal. They got a small slice of the action alongside some of EQT's larger backers and minority partner Santo Holding, the investment vehicle of the German Strüngmann family. It was an unconventional approach in the booming co-investment market which recognised that small investors, as well as large pension funds and sovereign wealth funds, are increasingly after a portion of the deals that buyout firms do.