The number of e-retailers in the Internet Retailer Top 500 Guide that aren’t large or small, but fall into the midmarket range has been growing steadily since 2008. That year, 172 retailers in the list had annual web revenues between $20 million and $75 million, making them mid-tier e-retailers by the definition of e-commerce consulting firm FitForCommerce. In 2013, the group numbered 251—more than half of the Top 500.

As more small retailers grow into the midmarket range, they’ve had to adjust their strategies to compete with the likes of Amazon.com Inc. and other industry stalwarts. To do so, they need to offer highly customized web site features that make them stand out and keep shoppers coming back.

While about a third of the midmarket retailers in the 2013 Top 500 Guide still rely on their own personnel to provide that level of customization, others are turning to the growing number of software vendors offering feature-rich e-commerce platforms at increasingly affordable prices. The most popular providers are those developing technology specifically for the middle market, says Kerry Martin, FitForCommerce senior consultant. That means offering much of the same functionality as the world’s largest retailers can afford, but with more flexibility, simpler interfaces and at a significantly lower price tag, she says.

Vendors catering to midmarket retailers include some that sell software almost exclusively to midmarket retailers, such as Micros Retail/Fry and Kalio Inc.  Others are known for having more clients on either the smaller side (Yahoo Inc.) or at the high end (Oracle Corp., GSI Commerce/eBay Enterprise or IBM Corp.).

Some of the vendors that serve large e-retailers have been retooling their software to make it more suitable for fast-moving, budget-conscious mid-tier e-retailers. For instance, some are hosting their platforms online and letting clients access it via a web browser, a model known as software-as-a-service. That frees retailers from having to maintain their own data centers and the software on their own. Forrester analyst Martin Gill in the 2013 report, “Selecting Tools that Enable Agility,” points to IBM and Oracle as examples of large vendors that are increasingly offering software-as-a-service versions of their e-commerce platforms  to make them more appealing to midmarket clients. That’s increasingly attractive to retailers that want to launch new stores quickly or require frequent technology changes, Gill says.

The on-demand model can also be attractive to large retailers that want to be able to launch e-commerce quickly in a new market. For example, Williams-Sonoma Inc., the housewares retail chain that maintains its U.S. e-commerce platform in-house, used web-hosted software from NetSuite Inc. to launch an e-commerce site in Australia last spring. Chief information officer John Strain says it took only three months to go live with NetSuite in Australia with features that would have taken the retailer three years to build on its own.