Math & Science ⇒ Fractions too big or too small

[Cham]

Most of the times, the default fraction command in math mode is of the proper size. But frequently, the fractions are too big. Or course, I could use the "text style" command \tfrac, instead of \frac or \dfrac, but then the fraction is too small.

How could I define something in between ? I mean a smaller fraction than the default, but not as small as the text mode style ?

And is it a good idea to use non-default fractions in math mode, under a display environment ? Compare the following three examples, made with the above commands (I don't think we need a MWE code for this example here) :

fractions.jpg (18.18 KiB) Viewed 15621 times

I find all three equations "disgusting" and I don't know how to display them.
The first equation is clearly too big for what it's worth.
The second one looks better, but it's weird in some way.
The third one is ugly.

Any suggestion ?