Part of the confusion may derive from the "fuzziness" subject. Time is not strictly rational in the Greek myths, so why should proportion be firmly fixed? More confusion may be injected in regarding all, as opposed to merely some, of the Titans, being huge. Finally, there is the question of who is and who is not a Titan, and according to whom?
When regarding the size Greek gods, stature influences conception. Zeus is towering per his power, but we regard him as dwarfed by the chthonic giants. Representationally, in the case of the metope, Atlas is of lesser stature than either Athena or Heracles, and representing him as larger would give the wrong impression. Remember that the idea of perspective in art is just starting to develop, and in the dominant tradition, commonly seen in Egypt, relative size of a subject is a function of importance. Note the normal size of the Erymanthian Boar in a reconstruction of another metope from the Temple of Zeus, and this vase depicting the same image. Yet the boar was said to be monstrous in stature, as depicted on this vase. I don't think it's coincidental that the boar is larger when a living threat than it is once vanquished.
Titans are not all the same size
Atlas is assuredly gigantic:
"...crafty Atlas, who knows the full depths of the sea, and holds, himself, the towering pillars which hold apart the earth and sky."
Source: Homer, Odyssey 1.52-54 | English
Unless we conceive of Atlas through the lens of Sun Wu Kong, who flits to the depths of the four seas on a whim, the implication is that Atlas knows the full depths of the seas because his feet rest on the bottom.
"Atlas through hard constraint upholds the wide heaven with unwearying head and arms, standing at the borders of the earth"
Source: Hesiod, Theogony 517-517
When we combine these two early descriptions, it's easy to see why Atlas is regarded as mountainous.
The association with pillars is further reinforced in the architectural atlantes. The lexical entry from the Liddell/Scott reads: Ἄτλαντες, in Architecture, colossal statues as supports for the entablature." This is, in part, a commemoration of Atlas as benefactor, turned to stone to keep the sky from crashing down to earth.
But it's hard to regard such Titans as Leto or Themis as gigantic. (Themis highlights another issue in that she is tricked into making herself tiny so that Zeus, himself a master shapeshifter, can swallow her.)
Epimetheus, generally regarded as the husband of Pandora, must have been man-sized or there would have been problems in the marital bed. (The presumed difference in the size of Loki and Angrboða, by contrast, would seem not to present such difficulties, and calls to mind a certain Almodóvar film;)
Who is and who is not a Titan, and according to whom?
We're talking about a canon that forms over a millennium in textual form. Later mythographers begin to conflate the Titanomachy and Gigantomachy, and giants with Titans, and the only thing constant is the state of flux.
Creating confusion is the existence of a Titan Pallas and a Giant Pallas. These may be conflated as the former comes from Hesiod, and the latter from Apollodorus, many centuries later. The Apollodorus further conflicts with Euripides' account of who was flayed by Athena during the battle. It's not that one author is right and the other wrong so much as each storyteller has their own take.
There's the question of Typhon. Not officially a Titan, but as the progeny of Gaia and Tartarus (compare to Gaia and Ouranos), Typhon is certainly a similar category of being, with a similar cycle of striving against the Olympians abd being cast down. "In size and strength [Typhon] surpassed all the offspring of Earth." From a modern perspective, influenced by the Harryhausen classic "Clash of the Titans", it's not difficult to see how a being like Typhon, or, in this case, the Kraken, might come to be thought of as a Titan. This may shed some light on both the confusion arising from ancient sources, and the exacerbation of that confusion in the present era per often poorly sourced or misleading information, and suboptimal translations.
In the early accounts, what seems to distinguish Titans is not their size but their early genesis and primal nature.
Hopefully this provides some answers on the question of Atlas in particular, and the Titans in general. Although I heartily agree that not all Titans would have been gigantic, and there is a similar example in Norse mythology, where not all giants seem to be the same size.
The association of great size with the Titans most likely relates partly to Atlas in his role as reluctant, but important, benefactor to mankind, and to Titans like Oceanus and Tethys, who, by their very nature, are titanic indeed.
No one who has witnessed the dawn breaking would conceive of "rosy fingered" Eos as anything but horizon spanning, and even when picturing her in human form, this association would be present. Homer is not being entirely literal when he describes Eos in her chariot riding to Olympus to hearken the day. Homer was a poet first and foremost.
The Titans are widely anthropomorphised, but these primal entities are also at once the things they represent, such as oceans, mountains and the dawn. This parallels the conception of Gaia and Ouranos as literally the earth and sky.