They have a relatively small surface area to volume ratio, so relatively little transport occurs, limiting their biochemical processes.
Imagine a cell as 10 by 10 by 10 units across (and ignore the fact it is a cube for the moment). It has a volume of 1000
It’s surface area to volume ratio is 60/1000 = 0.06
Now imagine a cell 2 units across: it has a volume of 8 units cubed and an area of 24 units squared, so has a surface area to volume ratio of 24/8 = 3 ..... much bigger.
So smaller cells are more efficient.