It's not so much the size of the heart but if it seems to be enlarged abnormally or simply a healthy looking heart that is large in size. My Bobo has a big heart all over, but the left side is dilated some as he has a heart valve defect that affects it. My Baz has a huge heart but the cardiologist says it is very healthy and he's just a big cat with a big heart.
I think it wise to have the ultrasound, it could easily be nothing but if it IS something, it is better to know early on as medication can make all the difference! When my Bobo was diagnosed with his defect at 18 months of age he was in congestive heart failure yet was seemingly asymptomatic. He was running and jumping and playing.. and had had four vet visits before in his life that had not heard the murmur at all.. it was a routine health check that found a murmur and lead us to xray and then take for a cardiac ultrasound. His heart condition was that serious they didn't think he'd make 2 years of age, yet here we are past his ELEVENTH birthday and he's going strong! We've carefully managed his medications and made adjustments and he's had a fabulous life quality (even if he still detests being pilled).
That's not to say anything is wrong with Loki, as I said with Baz he just has a big heart... but I wanted you to know that even if there is something to worry about, it can be managed and you will have Loki for years to come. I know Donna Lawver had her Guy with HCM for I think 12 years too...
I would be interested in seeing the spreadsheet...
. It will even let you "use measurements since..." if you want to know only what his last month's growth looks like.