For the same enlargement factor, at the same aperture, any lens of any focal length will produce the same exposure time. That's not the issue, though you might want a longer lens if your current lens doesn't cover the format adequately (i.e., if you're getting dark or unsharp corners).
I'd agree with those who say there is nothing unusual about your results, the neg is probably thin and will need a higher contrast grade, and use a smaller aperture to increase exposure time. Indeed, it's not optimal, but it sounds like the print isn't at the stage where diffraction is anywhere close to the main problem. When it gets to that stage, then you might worry about enlarging at the optimal aperture of the enlarging lens, presuming the enlarger is aligned properly.