If the issue is being not big enough at the *exit* point, couldn't you carry a small amount of extra fuel, which could be squirted out into an "emergence bubble" just as you cross the boundary, thus making your too-small ship 100dt in volume for that bare instant?

This would also explain why jump ships smaller than 100dt are rare; they're necessarily less fuel efficient than a standard design, so their operating expenses are increased. Even so, they are probably more efficient that designs using inflatable balloons or even retractible wings to make up the size shortfall, since an "emergence bubble" requires much less in the way of dedicated design space or precision engineering. Rather than complicated folding machinery (which can jam), the extra fuel could be carried in small tanks, tucked into dead spaces scattered throughout the ship's frame.