I copy and paste the Mipav manual page 61:

When you run MIPAV for the first time you must specify the amount of 
memory that you would like to allocate, so that MIPAV can take the full 
advantage of the memory resources available in your machine.
1 Select Help > Memory allocation in the MIPAV window. The Change 
JavaRuntime Memory Allocation dialog box opens. See Figure 6.
2 Change the maximum heap size number so that it reflects the maximum 
amount of memory that should be allocated. 
In a Microsoft Windows system, you can specify a maximum to 1.2 or 1.3 Gb. If 
additional memory is needed, MIPAV allocates it until the memory reaches the 
maximum heap size that you specify.
3 Click OK to close the dialog box.
You must restart of MIPAV for the changes to take effect. To exit MIPAV, 
select File > Exit. Then, restart the software

I hope this solve your problem
Riccardo Biffi