However, the obvious symptoms may just be excess width as the curved sides may be cut off by the CRT bezel.
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In particular, this sounds like a pincushion problem - to correct for pincushion, a signal from the vertical deflection that looks something like a rectified sinewave is used to modify width based on vertical position. There is usually a control to adjust the magnitude of this signal and also often, its phase. It would seem that this circuit has ceased to function.
If you have the schematics, check them for 'pincushion' adjustments and check signals and voltages. If not, try to find the 'pincushion' magnitude and phase adjustments and look for bad parts or bad connections in in the general area. Even if there are no adjustment pots, there may still be pincushion correction circuitry.
If the internal controls have absolutely no effect, then the circuit is faulty. With modern digital setup adjustments, then it is even tougher to diagnose since these control a D/A somewhere linked via a microprocessor.
Pincushion adjustment adds a signal to the horizontal deflection to compensate for the geometry of the CRT/deflection yoke. If you have knobs, then tracing the circuitry may be possible. With luck, you have a bad part that can be identified with an ohmmeter - shorted or open. For example, if the pincushion correction driver transistor is shorted, it will have no effect and the picture will be too wide and distorted as shown above.
However, without a schematic even this will be difficult. If the adjustments are digital this is especially difficult to diagnose since you don't even have any idea of where the circuitry would be located.
Faulty capacitors in the horizontal deflection power supplies often cause a similar set of symptoms.