Low readings

I had gd with my first so even though I haven’t been tested I’ve been taking readings since 12 weeks. I had a scan the other week and babies tummy was measuring large so I’ve been put on metformin. But in the last 19 weeks I’ve only had 4 high readings otherwise I’ve been fine. I had Chinese takeaway for dinner (wasn’t exactly careful with what I ate) and it was only 5.7. Other day I had weetabix with banana and it was 4.8, I know a lot of people with gd can’t have weetabix as it spikes so high. I just feel like my readings are always very good so not sure why baby (may) be large.