First, "fruit with larger-length tops" can be rephrased as "fruit with large-length tops", or better yet, "fruit with longer tops". The comparative form larger isn't necessary in this case because your comparison is between 'normal' and 'large', not between 'large' and 'larger'. Likewise with the term smaller. Hence those in the middle can simply be called "fruit with medium-length tops".
Nevertheless, you raise an interesting question: large and small have comparative forms. Why doesn't medium?
The positive form (e.g. large) and superlative form (e.g. largest) are absolutes - they describe just one thing. Comparative forms (e.g. larger) are different; they describe the relationship between two things.
In the phrase X is larger than Y, X is measured against Y and asserted to have more of something, in this case, size. When used with just a single noun, such as in your phrase "larger-length tops", the comparative larger is used as if it were an absolute, but there is an implicit standard of normal-length tops or medium-length tops against which the larger-length tops are asserted to have 'more' length. Likewise for smaller.
Strictly-speaking, given two larger-length tops A and B, all we can say is that they have more length than a top of standard length. There is no assertion that A and B are of the same length. For example, suppose a top of length 10cm was standard (I'm making this up). Then tops of length 20cm and 30cm would both be said to be 'larger' than standard.
When we come to *medium-length tops", there is no equivalent notion of degree. If two tops are medium-length, they are of the same size as all other medium-length tops. Since it is an absolute, strictly-speaking there is no sense in which one top is 'more medium' than another. As such, there is no practical use for a comparative form for the word medium. Conceptually though, the comparative 'form' would be more medium.