If the body size of an ectotherm with a semi-spherical body shape were to increase considerably, the surface area-to-volume ratio would , and the animal's ability to exchange heat with the environment would .

decrease

decrease.

Explanation:

The larger the body volume of an organism, the higher the amount of internal  heat generated.Likewise the higher the mass of the organism, the lower the surface area available to lose heat.

Thus the greater the surface area to volume ratio available to  an organism in its environment, the larger the quantity of internal  heat to be dissipated in proportion  to its volume.

Development in ectotherm  increases the  body volume cavity at  faster rate than the surface area, thus the ectotherm will have low surface area to volume ratio.Therefore the surface area  available to dissipate heat to the surroundings  decreases.

Thus  the ectotherm,  with  decrease  surface areas to volume ratio lose less  heat , because of limited/ decreased surface area exposed to the surrounding due to increase body volume.

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