A couple of thoughts:
First, consider a smaller venue - say for about 80-85 people. This will make the event seem "bigger" to new attendees and help you generate some buzz about it being well-attended. A large, empty space that's half-full creates the opposite impression - even though 75 people is a really decent turn-out.
Second, I'd put the question of how to increase turnout to your attendees via a web survey asking whether they would recommend the event to other entrepreneurs that they know and why/why not. This gives you a net-promoter score that you can use to gain some insights on what folks find most valuable which in turn can be used to target new attendees.
Great question, by-the-way. Good luck!