I need some specifics before offering an answer. Large and uneven foam structure is usually caused by early initiation of the blowing agent or poor distribution of the additive.
What is your resin/polymer system?
Processing temperature or polymer melt temp @ injection.
Are you using a M/B or straight blowing agent?
What type of blowing agent (azo, Bi-carb, etc.) are you using?
Are you using a foam catalyst?
Are you using a nucleating agent?
Or are a catalyst and/or nucleating agent included in your foam M/B?
There are ways to hasten or retard the "blowing" action of specific blowing agents. Your blowing agent supplier may be a good place to start for help.