Different Request Item Counts Between Import-Merge and Defined Load Using the Same Source File

I am testing a defined load against a viewpoint containing 85K+ nodes using a complete dimension file. With Import-Merge, the load processes successfully and generates a single request item. However, using the exact same file in a Defined Load configured with Replace mode results in more than 20K request items being generated. Has anyone encountered this behavior before, and can explain why the request item count differs so significantly between Import-Merge and Defined Load using the same source file?