It's all in the measurements. If everything but the heads remain the same, the extra cc amount in the larger chambers of the 340/360 head reduce the compression. A nominal cc amount of a 340/360 head is 72 cc's. However, they can be 2 - 4 cc difference from head to head.
In order to maintain the compression ratio of between the 2 heads, the larger chambered head must be milled down to match the smaller chambered head.
Less area to squeeze in the same volume of air and fuel intake when the piston goes up will raise the ratio.