The initial phase of construction (what DC Blox initially applied for permits for) is for 69k square feet at 10 MW.

But that is only a corner of the property that DC Blox acquired. Other documents filed during the permitting process include their long-term plan for the property, and that's where the 50 MW number comes from-- two buildings totaling 345k square feet of data center space, a 72k square foot electrical substation, plus support facilities (parking, guard houses, etc.).