Excessive AVR static RAM usage with mixed-type math operations
For many mathematical functions there's a trade-off between RAM usage and speed. I guess the AVR floating-point library will be optimized for speed.
Bitrex is right: try to avoid floating-point in microcontrollers unless absolutely necessary. Do you really need floating-point, or will fixed-point do? You can emulate fixed-point by placing a virtual decimal point somewhere in an int.  

In your example you multiply samp by \$\frac{2}{3}\$. The result, stored in a BYTE won't have a very high precision. In a case like that you can pick a scaling factor which is easier to calculate than the \$\frac{2}{3}\$. Find an approximation of the form \$\frac{A}{2^N}\$. In this case  \$\frac{171}{256}\$ ~ 0.668 is a good choice, with an error of only 0.2%. And it's a lot easier to calculate: multiply by 171, and shift right 8 bits. No division needed.