Trouble making decent liposomes from a mixture of phospholipids

I've been trying to make liposomes from a 1:1 mixture of two phospholipids with saturated alkyl chains. The chain lengths are the same for both the lipids. However, the polar heads have been functionalised with different groups.

Each type of phospholipid on its own forms small vesicles with good PDI (<0.15). It's when I mix the phospholipids together that I start having trouble (PDI of around 0.4). Would adding cholesterol to the system make it better? Thanks.