how to get leathers BIG ???

Hi, every1 !!!. I am new on the forum but have the past 40 years kept mostly rare freshwater fish/plants but also certain marine fish (I like species tanks). Have also kept some sps/lps corals but I still have some anemones ~ my babies (favorites !!!). The anemones have kept for 7 yrs + now. 2 years ago I decided to get some good size leathers (+ small frags), put them in 2 x 200 L (40 gal) tanks ~ 30cm/12in deep. They are connected to a same size refugium (200L, filled with coralsand 3in deep and lots of cheato, aslo mechanical filtersponges). Total is 120 gallons. I don't use any activated carbon or phosphate remover. Fish are only maroon clowns. Light at the moment is (tank 1) 100% sunlight (shaded with Spectranet Blue net) and (tank 2 and refugium) 6500K/1.2 watt per gallon. I use REDSEA CORAL PRO salt and all my parameters are perfect. NOW, MY QUESTION IS: how do I get leather corals to grow BIG ? (and as fast as possible ?). The reason why I am asking is; I have no experience growing leathers and I would like to set up a BIG tank with BIG leathers in asap. My experience the past 2 yrs have shown that they r pretty slow growers ~ very fustrating !. Except that they are growing slowly, they are he very healhy and not lost any I have bought the past few years. My anemone collection (haddoni's, mini max's, condy's, ext) is in exactly same type of setup and have been doing great for 7yrs + now. Any advice will be much apreciated ~