Worn magic items do resize
The Dungeon Master's Guide on Size and Magic Items says
When an article of magic clothing or jewelry is discovered, most of the time size shouldn’t be an issue. Many magic garments are made to be easily adjustable, or they adjust themselves magically to the wearer. As a rule, size should not keep overweight characters, characters of various genders, or characters of various kinds from using magic items. (213)
...And the DM really should rule that magic items that change size to fit their wearers also change weight so their wearers can wear the items and still move.
Extrapolating from how armor weight changes based on the size of the creature the armor's made for (Armor for Unusual Creatures on Player's Handbook 123), a healing belt (MIC 110) (750 gp; 1 lb.) should weigh only 0.1 lbs. when it resizes to fit a Diminutive creature (like a bat or toad).
Beyond this, with DM approval a healing belt could—instead or in addition—be made from a special material lighter than its normal material, but materials lighter than the belt's traditional leather are difficult to find. (For example, leafweave (Arms and Equipment Guide 20) is said to be lighter than leather but the weight of armor made from that special material isn't reduced.)
Note that the more difficult task than acquiring such an item is convincing the DM that, even if the creature's given the ability to utter the command word by a pearl of speech (MIC 118) (600 gp; 0 lbs.), the nonhumanoid Diminutive creature can benefit from the item in the first place. The DM may rule the creature just can't wear a belt. The Random Encounters: Wild Life Web column "Tricks and Training for D&D Animals" says some animals can use a "belt or strap worn in front of or over the haunches," but ask the DM if that includes the creature in question.
This may mean commissioning a custom double-price slotless version of the healing belt—an ioun stone (healing), maybe?—, which would probably be nearly as expensive as any special material version of the belt would've been anyway and eliminate completely concerns about weight.