Some stocks might seem too big to generate explosive growth. Others could be so small that their risk level isn't appealing. However, similar to the old story of Goldilocks and the three bears, investors can find stocks that are just right -- not too big and not too small. 

Mid-cap stocks can meet those criteria. These stocks, which are usually defined as having market caps between $2 billion and $10 billion, often provide a sweet spot for investors looking for tremendous growth prospects with less risk than most small-cap stocks have. 

But which of these Goldilocks kinds of stocks are smart picks now? Here are three top mid-cap stocks to buy in September.

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Fiverr's (FVRR 2.13%) market cap of around $6.5 billion fits right into the mid-cap category. The company provides an online platform that connects freelancers with organizations seeking to farm out digital projects.

The stock has fallen more than 40% from its peak set earlier this year. This decline is partially due to the shift away from growth stocks that began in February. However, Fiverr's shares also plunged nearly 25% in early August after the company announced weaker-than-expected third-quarter revenue guidance.

Fiverr still expects revenue to jump between 30% and 38% year over year. That would be great for most companies but not for Fiverr. The company attributed the slowing growth to "reduced online activity" with COVID-19 restrictions being lifted in many areas.

This slowdown seems likely to be only a temporary one. The freelance market should continue to boom over the long term. Fiverr is currently just scratching the surface of its global market opportunity. With new products and services that should attract more freelances and businesses, I think the stock has the potential to be at least a five-bagger over the next decade.  

Innovative Industrial Properties

Innovative Industrial Properties (IIPR 2.20%) also could be in the sweet spot for many investors with its market cap of nearly $6 billion. The company is a real estate investment trust (REIT) focused on the U.S. medical cannabis industry.

Unlike Fiverr, IIP has been sizzling hot so far this year. Its shares have soared more than 35% year to date. The company continues to beat expectations quarter after quarter. In Q2, IIP reported revenue that more than doubled year over year with earnings per share up 60%. 

Investors also like IIP's dividend. Its yield currently stands at close to 2.3%. That's lower than it would otherwise be because the stock has risen so much. IIP has quadrupled its dividend payout over the last three years.

The REIT should have strong growth prospects ahead as well. IIP only operates in half of the states that have legalized medical cannabis. More states could launch cannabis markets over the next few years. 

Scotts Miracle-Gro

You might be somewhat surprised that Scotts Miracle-Gro (SMG -2.39%) is a mid-cap stock. Its consumer lawn and garden products are household names across the U.S., and the company has been in business since 1868. But its market cap is only around $8.7 billion.

Scotts' shares are down nearly 40% from the highs set earlier this year. The company's growth is slowing. However, that's to be expected considering the stunningly great performance Scotts delivered in 2020 with many people at home due to the COVID-19 pandemic.

The key growth driver for Scotts Miracle-Gro these days is its Hawthorne unit, which ranks as a top supplier to cannabis growers. Hawthorne's sales jumped 48% year over year in the latest quarter and accounted for 26% of the company's total revenue. 

Scotts should enjoy a huge boost if federal cannabis reform is enacted that expands the U.S. cannabis market. That seems likely to happen sooner or later. CEO Jim Hagedorn said on the company's recent earnings conference call, "There is little doubt this industry is poised for significant growth." I think he's right. And I expect that Scotts stock should be poised for significant growth over the long term as well.