Kate is the flat envelope while LouLou carries volume
The Kate Small is published at 20 × 12.5 × 5 cm. The LouLou Toy is 20 × 14 × 7.5 cm, so it is only as wide as the Kate but 1.5 cm taller and 2.5 cm deeper. LouLou Small moves to 23 × 17 × 9 cm. That difference is enough to change the contents: Kate's 5 cm depth favours a phone, cards and flat keys, while LouLou Small's 9 cm body can take a compact wallet and a small pouch. The word Mini does not describe one Saint Laurent construction, so width alone is not enough.
Niki's depth is softened by its slouch
Niki Mini is listed at 18 × 12.5 × 5 cm, close to the Kate in paper dimensions. Its quilted body is softer and its leather-through-chain strap behaves differently when the bag is full. The published depth describes the body, not how much the front can move once the flap closes. That makes Niki a poor candidate for a rigid oversized object, but a practical one for a phone, card holder and keys because the contents settle against the body. Ask for a side view with the bag filled if you intend to carry a compact wallet rather than only cards.
Strap drop separates evening and crossbody use
LouLou Small publishes a doubled drop of 30 cm and a single drop of 56 cm. The Toy publishes a 57 cm shoulder drop and a detachable wristlet section. Those figures explain why the two nominally small bags sit at different heights: the longer single pass can cross the body, while the doubled chain hangs closer to the underarm. A 23 cm LouLou Small with a 9 cm body is therefore not too large for evening use when the chain is doubled, but it is a different line on the torso from a flat Kate. Ask for the chain measurement end to end as well as the drop if a listing shows only the strap.
The practical order for choosing a YSL mini
Decide first whether the bag must hold a full wallet. If not, Kate Small and Niki Mini keep the silhouette flat. If a compact wallet and pouch are required, LouLou Small is the only one of the three whose 23 × 17 × 9 cm body supports that load comfortably. Then choose the strap position: doubled or single. Finally compare material. Patent leather reflects light and shows a sharper edge, smooth calfskin creases more visibly, and quilted leather changes how the side profile reads. Those are visible properties, not a substitute for confirming the specific reference.
Reading the quilt pattern and hardware without guessing the model
The front face is often the fastest model check. Kate carries a centred rectangular YSL plaque on a flat flap. LouLou has broad chevron quilting around a smaller metal logo. Niki looks softer through the body and the chain is threaded with leather, so the strap is less metallic from a distance. A catalogue photo can combine those cues with the side profile: a straight gusset points toward a structured flap, while a body that collapses below the flap points toward a slouchier construction. Still confirm the printed reference, because the same pattern can appear in more than one size. Matching the geometry first and the label second is more reliable than choosing from colour alone.