What (I think) you're talking about are either boutique (on the 15 lawyer end of your spectrum) or what I would call "premium midsize" (on the 75 lawyer end of your spectrum). The boutiques tend to be highly specialized litigation shops; premium midsize firms tend to have mostly "middle market" clients (e.g., local companies, or they handle the local/regional affairs of national/international companies). Both (generally speaking) tend to have hours requirements of around 1850, rather than the 2000 standard. Total compensation (generally speaking) at a boutique can equal or exceed biglaw; total comp at premium midsize firms tends to be slightly less than the best biglaw firms (but note also that partnership prospects tend to be better at a premium midsize firm).
Both tend to hire mostly laterals from biglaw. (Where do you think 50% of biglaw associates, who leave their firms between years 3-5, go? For those who don't go in-house, to the government, to academia, or into politics, boutiques and premium midsize firms are the destinations of choice.) However, boutiques and premium midsize firms do hire a very limited number of summer associates (who then return as associates after graduation). But note that they are very selective (i.e., those hired at such places tend to have multiple offers from biglaw firms).
If you're an editor of the law review at a top (i.e., top 6) school, after your prestigious federal clerkship, you might find yourself starting out at a boutique. Otherwise, you might lateral to one after a few years at a top biglaw firm. If you're merely on the law review at a top (i.e., top 14-25, depending on the market and just how "premium" the midsize is) school, you might land one of the three or four summer associate positions at a 75-100 lawyer firm. Otherwise, you might lateral to one after 4-6 years and a good biglaw firm (this will be a highly practice area-driven outcome).
In sum, it's really not an "either or" situation, i.e., if you are eligible for a job at a boutique or premium midsize (either as a summer and then first year associate or as a lateral), you will also have multiple offers in hand from the leading biglaw firms in your desired market.