I was confused by this until just now, so here's how I've understood this.

Size indicates the size of a specific order.

Sum gives you the total size of a currency available for that price.

That is, in the image below:

• if you are will to pay up to 68 XRP per USD, there are 2,451.566242 USD available for purchase.
• if you are only willing to pay up to 67.50 XRP per USD, there will only be 100 USD available for purchase.

As you move down the chart, the "Sum" corresponding to an order is the sum of all the sizes up to an including that order.

My pseudo economic analysis: Sum is an necessary indicator of the true price. If the cheapest Ask order has Size 0.5 USD, that ask price doesn't tell you much.