What is the setting of PCTFREE for those tables?

Let's say it was set to 60. So after initial inserts all those table blocks were only 40% full. Then you made some updates to those rows which might fill those blocks completely. So your tables occupied 50 megs of space with all their blocks (almost) completely full before you performed an export, although their PCTFREE was set to 60%.

Now when you import them back, imp filled the blocks only up to 40% full because the PCTFREE is still set to 60 for those tables. That mean that imp will need at least 125 megs of space to import those tables back.

Could this be an explanation for your problem?