This chart shows one big reason why Verizon just spent $4.4 billion on AOL

Verizon announced on Tuesday that it's acquiring AOL for roughly $4.4 billion.

At a morning conference for analysts, John Stratton, head of Verizon's operations, said its "principal interest was around the ad tech platform that Tim Armstrong and his team have done a really terrific job building."

So how big is AOL's ad tech business? According to the chart below by BI Intelligence, AOL's third-party platforms, which include programmatic and traditional ad marketplaces, is about a third of AOL's total business. It accounted for 37% of total revenue in the last quarter, up from its 24% share just two years ago. Its publishing business, which includes big-name brands like Huffington Post and AOL.com, still comprise the bulk of the overall business, but the ad tech side is growing at a faster clip.

Verizon has seen its business slow down as its core mobile broadband and internet traffic turn into "dumb pipes," BI Intelligence writes. The AOL acquisition instantly gives Verizon a new revenue source that could potentially make Verizon a major digital-advertising player in the ad tech space.