The open SSID with a splash page may attract more passersby looking for free Wi-Fi, which may explain why you're seeing more data for this SSID. The PSK-WPA2 SSID, on the other hand, is probably only being used by people who have the correct password? Which is probably a much smaller group.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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