I think your basic premise is wrong. Let's have a look at the data:
Questions per day 13.2 19.4
Percent answered 91% 91%
Avid users 491 347
Total users 8,727 8,025
Answer ratio 1.5 1.5
Visits/day 9,852 15,769
The only difference that could conceivably be significant is the number of visits per day. The rest are basically the same.
The same goes for your Quantcast graph. I see no significant difference there. The reputation gained by the quarter's top user is not really useful information. That could just suggest that the top user on Chemistry is more active than the top one here. What you should look at is total reputation gained by all users in the last quarter. Since I don't have the time to figure out how to get that information, here's what the top 20 users gained in the last quarter:
Chemistry : 49200
Biology : 35734
OK, so Chemistry users gained 13466 more reputation than Biology users in the last quarter. In order to understand what that means, however, one has to take into account the number of "voteable" posts, i.e. answers & questions (graphs generated using this SEDE query):
The actual data (this is for the period 2014-12-16 to today, 2015-03-17) are:
Votes per day 101 123
Questions per day 10.3 15.2
Answers per day 12 20
So, in the last quarter, Chemistry has indeed been a little bit more active than we have. On the other hand, we've had a total of 9117 "voteable" posts (questions and answers) while Chemistry has had 11097. Since they have more posts, it's reasonable that they also gained more rep as we saw above. What is encouraging though is that we have an average of 4.5 votes/post while they are at 3.5. This means that they get slightly more questions per day but we vote more per question.
In conclusion, we can say that while Chemistry might be slightly more active than we are, the difference is very small and we seem to be better at voting. All things considered, I don't see an issue here.