If your wedding is over 150 people and your parents paying let them invite them. When you’re having a large wedding you can’t use the excuse that you want an intimate one.
Since you’ve accepted money from your parents, and I’m assuming the groom’s too, you have to accept some strings.
Remember that your wedding day is just as important to your parents as it is to you. In fact I’d bet they’ve been thinking of it for longer than you have. Just as you want to share it with your friends, they want to share it with theirs.
Review the list with your parents and if money is not a point of debate because your parents are willing to pay the added cost (catering, invitations, cake, rentals, favors, flowers for five more tables are some of the added costs you can mention), think of the space. Can it accommodate another 50 people?
Go over each name and if you’re not sure who someone is ask who he or she is and why it’s important for him or her to be there. Don’t have an edge in your voice, just speak quite frankly.
If on the other hand you’re only having immediate family and your closest friends explain to your parents that the style of wedding you and your fiance want won’t accommodate more people and maybe when you return from your honeymoon there can be a post-wedding reception: a brunch, cocktail party or formal dinner.