My ultrasound report showed a mild enlarged liver (15cm) and the rest of it came normal with normal lfts. one month later another ultrasound showed a normal liver in shape and size. should i be worried with that slight increase in size previously?

Only couple of possibilities: either an acute condition causing a transient liver enlargement vs an overestimate at the first read providing the second test is correct. A follow up ultrasound will be needed, ideally same machine, same operator to clarify trend. Your treating doc remains your best reference to place findings in proper clinical context.

All imaging results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that. Talk to the doctor who ordered the tests. Having said that, the slight increase per se is not a cause for concern. If you are overweight, losing weight would be helpful in restoring your liver health. Wish you good health!