English articles

Hello, 

There are several sentences: 

Experts have found that the smaller the mosquito the more likely that it is infected with viruses which cause human diseases. 
And in the world of micro-electronics, the smaller a circuit, the more circuits a chip can hold. 
Those extra costs can be recouped in energy savings, but the smaller a project is, the more time it may take. 
The smaller a star is, the longer it will live. 
The smaller the farm, the better the food. 
In general, the smaller the aperture (the larger the number), the greater the distance from the plane of focus. 

Tell me please, why "the mosquito, the farm, the aperture", but "a circuit, a project, a star"? 

Thank you.