Hi - my 2yr old was in the obese range, however! And this was the critical piece, they were born 98% percentile. In their red book, they had tracked exactly as they should against the line.
HV look for large rising and large falls - that don't track against the line because that indicates an external or medical issue.
For example, if you have a child born 50 per centile and at the 2yr review they are 25 per centile that indicates that there could be an underfeeding or medical issue. Likewise if that child rises to 75 per centile it indicates an over feeding or again medical issue. These issues don't automatically mean - 'bad parent' but it merits a discussion.
As my child was born 98% then this just automatically put then into the obese range at 2yrs but the HV took one look at the graph and said - absolutely exactly where they should be and we were sent away. I was worried about it, so was asking lots of questions. HV was very clear that they are only interested in cases where a child has dramatically 'left the line'.
Between 2yrs to 4yrs old DC dropped from 98% to 96% to 93% - very very slowly over 2yrs (so not dramatic).
Then they started school, had a massive growth spurt and dropped to 81%. Their weight hadnt changed, but their height did. A drop from 93% to 81% by age 5 is not 'dramatic' when their height has shot up.
Now we have a healthy per centile child who just has a stocky build but is a perfectly healthy weight.
So look back in your red book you can see and calculate where your child should be weight wise based on their birth rate and other weigh in checks.