Small pet habitats sold as complete starter kits are frequently undersized relative to what welfare-focused guidelines actually recommend for the specific animal — worth knowing before assuming a prominently marketed "starter kit" reflects appropriate housing.
Bar spacing must prevent escape for the specific species
Similar to the bird cage sizing principle, wire spacing needs to be tight enough that the specific animal can't squeeze through or get its head stuck attempting to — small hamsters in particular are notorious escape artists capable of fitting through surprisingly small gaps. Checking species-specific bar spacing recommendations, rather than assuming a cage marketed for "small pets" generically has appropriately tight spacing for the smallest or most escape-prone species, is worth doing directly rather than assuming any small-pet cage is safely spaced for every small pet.
Many starter kits are smaller than genuinely recommended minimums
Commercially sold "starter kit" cages, especially budget ones, are frequently smaller than the floor space actually recommended by small-animal welfare organizations and experienced keepers for a given species — a hamster or guinea pig genuinely needs meaningfully more floor space to move, forage, and exercise than many prominently marketed starter cages provide. Checking independent, species-specific minimum size recommendations, rather than assuming a cage marketed as appropriate for that animal meets a genuine welfare standard, is worth doing before purchase rather than after the animal is already settled into an undersized space.
Solid-bottom vs. wire-bottom flooring
Wire-bottom cage floors can cause real foot and joint problems for many small animals over time, particularly rabbits and guinea pigs, whose feet aren't well suited to standing on wire mesh for extended periods. A solid floor, with appropriate bedding on top, is generally the safer, more comfortable choice, and wire flooring — if present at all — is best limited to small platform or ramp areas rather than the animal's primary standing and resting surface.
Ventilation and material affect long-term air quality inside the habitat
Enclosed plastic tank-style habitats can trap ammonia from waste and humidity more than wire cage designs with more open airflow, which matters for respiratory health with less frequent cleaning, or in a habitat that's already on the smaller side. Adequate ventilation, alongside a genuinely appropriate cleaning schedule, is a real health factor, not just an odor-control consideration.
The one thing people forget
Provide a genuinely enclosed hide or shelter area within the habitat, regardless of species, giving the animal a private space to retreat and rest away from constant visibility and activity — a real, well-documented stress-reduction factor for small prey animals in particular, who instinctively seek concealment, not just a decorative habitat accessory.